Testosterone makes a contribution to nitric oxide formation. Nitric oxide, released from penile nerves stimulates guanylate cyclase which catalyzes the transformation of guanosine-5-triphosphate into 3′,5′-cyclic, guanosine monophosphate (cyclic GMP). Gyclic GMP causes vasodilatation and hence erection formation (Morelli et al 2005). The breakdown of cyclic GMP to GMP is mediated by the enzyme, phosphodiesterase type-5, the inhibitors of which (eg, sildenafil citrate) enhance erection formation and maintanence (Carson and Lue 2005).
Kostic TS, Stojkov NJ, Bjelic MM, Mihajlovic AI, Janjic MM, Andric SA: Pharmacological doses of testosterone upregulated androgen receptor and 3-Beta-hydroxysteroid dehydrogenase/delta-5-delta-4 isomerase and impaired leydig cells steroidogenesis in adult rats. Toxicol Sci. 2011 Jun;121(2):397-407. doi: 10.1093/toxsci/kfr063. Epub 2011 Apr 6. [PubMed:21427060]
This paper will aim to review the current evidence of clinical effects of testosterone treatment within an aging male population. As with any other clinical intervention a decision to treat patients with testosterone requires a balance of risk versus benefit. We shall try to facilitate this by examining the effects of testosterone on the various symptoms and organs involved.
Smoking doesn’t promote maintaining male hormone levels healthy. The study has shown that smoking deprives the body from zinc. Zinc deficiency is dangerous for men because it is fraught with testosterone deficiency. The matter is that zinc is a kind of structural material for building the testosterone molecules. So, smoking combined with unhealthy diet strikes a blow against normal testosterone production.
BSN's Evotest is a top of the line testosterone powder designed by award BSN. While taste of this product has been suspect, Evotest aims to help pump you with naturally produced testosterone by improving your own testosterone production. Most people like the convenience of pills, but for those that can’t take pills, this powder might be a good alternative. If you want to avoid swallowing pills and boost testosterone levels, Evotest may be for you. Click To Read More...
“I'm a truck driver and for 13 hours a night I sit in my truck and I drive. Out of boredom, I'd stop and eat. That was all until Andro400 – ever since then my life has changed. I started out weighing 341 pounds, and since taking Andro400 I've dropped 85 pounds! There's no cravings – I actually don't even think about food anymore. One thing that Andro400 said on the radio ad is it attacks belly fat – well let me tell you it did – the 2nd month is where I saw a drastic change in the size of my stomach. I've lost 6 inches! I'm sleeping better. My knee pain went away. I've had some lower back issues and that went away, and I can only attribute that to Andro400. It's a Life Changer for me!”
“I'm 55 years old and hitting the ball further than I've ever hit, and I'm not getting tired going 18 holes! And when I play softball I'm hitting the ball further. I work for the DWP in LA and it's a very physically demanding job. Andro400 really helps because we work 16 hour days a lot. I was turning down a lot of overtime, but when I started taking Andro400, it got me through the day. I really notice a difference – even my wife did. It really works!” 

On the average, you need to sleep at least 8 hours per night to stay healthy. If you want a night sleep to contribute to the maximum testosterone production, it’s important to make your sleep comfortable. Thus, the bedroom temperature shouldn’t exceed 21°C. In addition, you should ventilate your bedroom thoroughly before sleeping. Furthermore, before going to bed, don’t overload your stomach with fatty foods, as well as don’t drink alcohol and caffeinated beverages. Finally, you have to avoid intense physical activity before bedtime.6
While it would be nice to buy a testosterone pill from the local supplement store and have your testosterone levels go up, such a magic pill does not exist. As you can see from the above rundown, while a few supplements may be somewhat effective if your T levels are already low, none will significantly raise your testosterone above a baseline level. Thus, the basics of keeping your T levels high remain pretty simple:

^ Jump up to: a b Sapienza P, Zingales L, Maestripieri D (September 2009). "Gender differences in financial risk aversion and career choices are affected by testosterone". Proceedings of the National Academy of Sciences of the United States of America. 106 (36): 15268–73. Bibcode:2009PNAS..10615268S. doi:10.1073/pnas.0907352106. PMC 2741240. PMID 19706398.

A number of epidemiological studies have found that bone mineral density in the aging male population is positively associated with endogenous androgen levels (Murphy et al 1993; Ongphiphadhanakul et al 1995; Rucker et al 2004). Testosterone levels in young men have been shown to correlate with bone size, indicating a role in determination of peak bone mass and protection from future osteoporosis (Lorentzon et al 2005). Male hypogonadism has been shown to be a risk factor for hip fracture (Jackson et al 1992) and a recent study showed a high prevalence of hypogonadism in a group of male patients with average age 75 years presenting with minimal trauma fractures compared to stroke victims who acted as controls (Leifke et al 2005). Estrogen is a well known determinant of bone density in women and some investigators have found serum estrogen to be a strong determinant of male bone density (Khosla et al 1998; Khosla et al 2001). Serum estrogen was also found to correlate better than testosterone with peak bone mass (Khosla et al 2001) but this is in contradiction of a more recent study showing a negative correlation of estrogen with peak bone size (Lorentzon et al 2005). Men with aromatase deficiency (Carani et al 1997) or defunctioning estrogen receptor mutations (Smith et al 1994) have been found to have abnormally low bone density despite normal or high testosterone levels which further emphasizes the important influence of estrogen on male bone density.
Testosterone boosters are a class of herbal supplements aimed at naturally increasing your testosterone levels. Usually, they contain micronutrients that men are commonly deficient in, such as zinc, and which have been connected in research to healthy testosterone levels. They also may contain adaptogens, which are a class of supplement that are thought to help the body adapt to stress, or ingredients which have been connected to improved sleep. Sleep restriction has been shown to reduce testosterone in healthy young men, and as Chris Lockwood, Ph.D., notes, disturbed sleep is a common symptom of low T-levels.[1]
There are no studies showing its effects on healthy males, but it has been shown to drastically improve testosterone in infertile males (ref 77). It's also packed full of minerals, so is a great superfood nevertheless. I use the Sunfoods brand. Make sure you buy from a quality brand, as there are a lot of poor shilajit products out there, also some have been shown to be high in heavy metals. 
So if you’re intent on maximizing your testosterone levels, and/or you have applied all of the above and you’re still not satisfied with your results (which would be surprising) then you could try the below. I will point out that some of these tips may not have the scientific evidence to back them up like the previous points, but I can assure you that either I have or do use them (and have positive results), or a client has used them with pleasing results, or finally it is such a new conception that there isn’t enough evidence to prove it one way or another.
Zaima, N., Kinoshita, S., Hieda, N., Kugo, H., Narisawa, K., Yamamoto, A., ... Moriyama, T. (2016, September). Effect of dietary fish oil on mouse testosterone level and the distribution of eicosapentaenoic acid-containing phosphatidylcholine in testicular interstitium. Biochemistry and Biophysics Reports, 7, 259–265. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5613343/

A: Testosterone products can improve a male's muscle strength and create a more lean body mass. Typically, these effects are not noticed within the first two weeks of therapy, but it is possible that he is more sensitive and responds well to the therapy. Some of the other more common side effects of testosterone patches are headache, depression, rash, changes in libido, acne, male pattern baldness, and increased cholesterol levels. This is not a complete list of the side effects associated with testosterone patches. Megan Uehara, PharmD

A previous meta-analysis has confirmed that treatment of hypogonadal patients with testosterone improves erections compared to placebo (Jain et al 2000). A number of studies have investigated the effect of testosterone levels on erectile dysfunction in normal young men by inducing a hypogonadal state, for example by using a GnRH analogue, and then replacing testosterone at varying doses to produce levels ranging from low-normal to high (Buena et al 1993; Hirshkowitz et al 1997). These studies have shown no significant effects of testosterone on erectile function. These findings contrast with a similar study conducted in healthy men aged 60–75, showing that free testosterone levels achieved with treatment during the study correlate with overall sexual function, including morning erections, spontaneous erections and libido (Gray et al 2005). This suggests that the men in this older age group are particularly likely to suffer sexual symptoms if their testosterone is low. Furthermore, the severity of erectile dysfunction positively correlates with lower testosterone levels in men with type 2 diabetes (Kapoor, Clarke et al 2007).

6)  Take Cold Showers:  Cold showers have been known to stimulate and boost testosterone production and improve metabolism, detoxification and brain function.  Start your shower with warm/hot water and turn it to cold for the last 30-60 seconds while pumping your muscles and creating a big shiver as your muscles contract.  That will help to boost internal heat and boost testosterone production.  This article will help you.
This herb, used for centuries in foods, even poultices, was reported in the International Journal of Sport Nutrition and Exercise Metabolism to have reduced body fat and improved total testosterone levels versus a placebo in a double-blind trial. Fenugreek may also be helpful if you feel your sex drive is on the wane, as other research has found it can boost libido. You can get it in curries (it’s used to flavor them) and teas, or as a supplement in TestroVax, by Novex Biotech, which promises to boost testosterone levels 42% in 12 days. (novexbiotech.com)
Men who watch a sexually explicit movie have an average increase of 35% in testosterone, peaking at 60–90 minutes after the end of the film, but no increase is seen in men who watch sexually neutral films.[43] Men who watch sexually explicit films also report increased motivation, competitiveness, and decreased exhaustion.[44] A link has also been found between relaxation following sexual arousal and testosterone levels.[45]

This is an important herb which has been used as therapeutic for centuries. It helps in improving sexual desires and boosts T levels. It is also useful in erectile dysfunction by raising T levels. People having normal T level don’t get affected by taking Tribulus. With the testosterone boosting qualities of Tribulus, this natural supplement works great for building muscle and gaining strength in the gym.

Findings that improvements in serum glucose, serum insulin, insulin resistance or glycemic control, in men treated with testosterone are accompanied by reduced measures of central obesity, are in line with other studies showing a specific effect of testosterone in reducing central or visceral obesity (Rebuffe-Scrive et al 1991; Marin, Holmang et al 1992). Furthermore, studies that have shown neutral effects of testosterone on glucose metabolism have not measured (Corrales et al 2004), or shown neutral effects (Lee et al 2005) (Tripathy et al 1998; Bhasin et al 2005) on central obesity. Given the known association of visceral obesity with insulin resistance, it is possible that testosterone treatment of hypogonadal men acts to improve insulin resistance and diabetes through an effect in reducing central obesity. This effect can be explained by the action of testosterone in inhibiting lipoprotein lipase and thereby reducing triglyceride uptake into adipocytes (Sorva et al 1988), an action which seems to occur preferentially in visceral fat (Marin et al 1995; Marin et al 1996). Visceral fat is thought to be more responsive to hormonal changes due to a greater concentration of androgen receptors and increased vascularity compared with subcutaneous fat (Bjorntorp 1996). Further explanation of the links between hypogonadism and obesity is offered by the hypogonadal-obesity-adipocytokine cycle hypothesis (see Figure 1). In this model, increases in body fat lead to increases in aromatase levels, in addition to insulin resistance, adverse lipid profiles and increased leptin levels. Increased action of aromatase in metabolizing testosterone to estrogen, reduces testosterone levels which induces further accumulation of visceral fat. Higher leptin levels and possibly other factors, act at the pituitary to suppress gonadotrophin release and exacerbate hypogonadism (Cohen 1999; Kapoor et al 2005). Leptin has also been shown to reduce testosterone secretion from rodent testes in vitro (Tena-Sempere et al 1999). A full review of the relationship between testosterone, insulin resistance and diabetes can be found elsewhere (Kapoor et al 2005; Jones 2007).
There are a lot of test booster blends out there. A lot of them are junk. I have tried to cover the most effective herbs above. As always, I recommend doing your own research and experiment to see if you notice an effect. If you would like one easy herbal solution I recommend starting with Mike Mahlers Aggressive Strength product purely because I have solid anecdotal evidence of its effectiveness. But again, supplements should be seen purely as that - a supplement to a healthy diet, plenty of sleep, hard training with adequate rest.
Now that we know chronic insulin spikes lead to lower Testosterone production, I hope I haven’t sent you running into the low carb camp! There are a few studies out there showing that long term low carb or ketogenic dieting leads to higher cortisol levels (especially with subjects who are training), and decreased testosterone levels (28 & 29). I have used low carb diets in the past with successful results (winning a national bodybuilding title), however the key is to use cyclical carb re-feeds. If you’re going to go on a low carb diet for whatever reason, be sure to work in a large carb reefed once a week.

At the present time, it is suggested that androgen replacement should take the form of natural testosterone. Some of the effects of testosterone are mediated after conversion to estrogen or dihydrotestosterone by the enzymes aromatase and 5a-reductase enzymes respectively. Other effects occur independently of the traditional action of testosterone via the classical androgen receptor- for example, its action as a vasodilator via a cell membrane action as described previously. It is therefore important that the androgen used to treat hypogonadism is amenable to the action of these metabolizing enzymes and can also mediate the non-androgen receptor actions of testosterone. Use of natural testosterone ensures this and reduces the chance of non-testosterone mediated adverse effects. There are now a number of testosterone preparations which can meet these recommendations and the main factor in deciding between them is patient choice.

Individuals with metabolic syndrome are at increased risk for developing coronary artery disease and diabetes mellitus. Predicting who might develop the metabolic syndrome would allow preventive measures to be taken in addition to weight control and other lifestyle modifications such as cessation of smoking and increased exercise. It is known that with decreasing testosterone availability in aging males there is an increase in fat mass and decrease in lean body mass (van den Beld et al 2000), there are disorders of insulin and glucose metabolism (Haffner et al 1996) and dyslipidemia (Tsai et al 2004). Kupelian and colleagues (2006) in analyzing data from the Massachusetts Male Aging Study demonstrated that men with low levels of testosterone, sex hormone-binding globulin, or clinical androgen deficiency, especially men with a BMI of greater than 25, were at increased risk of developing the metabolic syndrome and hence, diabetes mellitus and/or coronary artery disease.

The testosterone supplements are primarily used to enhance the body muscle mass. Various studies have shown that testosterone therapy is also crucial in development of the memory as well as concentration, increase the level of energy, and increase libido or sexual desire. However, use of high levels of testosterone booster can also contribute to the development of certain medical conditions and health problems. Two types are available on the market, legal boosters, and illegal boosters. There are also certain excellent natural testosterone boosters that are efficient and so not have side effects.
In fact, testosterone supplements might cause more problems than they solve. Studies have suggested a connection between supplements and heart problems. A 2010 study reported in The New England Journal of Medicine showed that some men over age 65 had an increase in heart problems when they used testosterone gel. A later of men younger than 65 at risk for heart problems and heart-healthy older men showed that both groups had a greater risk of heart attack when taking testosterone supplements.
Low testosterone levels can cause mood disturbances, increased body fat, loss of muscle tone, inadequate erections and poor sexual performance, osteoporosis, difficulty with concentration, memory loss and sleep difficulties. Current research suggests that this effect occurs in only a minority (about 2%) of ageing men. However, there is a lot of research currently in progress to find out more about the effects of testosterone in older men and also whether the use of testosterone replacement therapy would have any benefits.