Most people associate testosterone with excessive muscle development, but it actually affects countless bodily functions, and adequate levels are essential in both men and women. Achieving optimal levels of testosterone can have positive effects on mood, energy, libido, and sense of wellbeing in both sexes, and studies in men have indicated large impacts on improving insulin function, decreasing body fat, and improving muscle mass. 
What many don’t realize is that all steroid hormones, including testosterone, estrogen, and progesterone originate from the same source, i.e. cholesterol.
In the “hormone soup” created as the body converts cholesterol into its large array of downstream sex/steroid molecules, it can be extremely difficult to accurately predict where hormones introduced directly into the body are ultimately going to end up. For example, testosterone converts into estrogen through the action of an enzyme called “aromatase”. One of the difficulties with directly introducing testosterone into a man’s body is that much of it may simply convert into estrogen. Accordingly, many prefer to encourage their body’s own production of hormones by natural means.
In this regard there are many botanicals, natural substances and natural testosterone boosters that can be helpful. But it is important to be clear that in the world of testosterone boosters, hype predominates. There are many claims that are made, but the science backing them up is usually scant, obscure, or non-existent.
Yet there are some products, (particularly the one containing the fenugreek extract discussed below), that are backed up by studies, and have also produced marked, and consistent positive feedback from customers.
So..which botanicals should you consider using?
Botanicals used to increase testosterone include Fenugreek extracts, Tribulus terrestris, Maca, Panax and Siberian ginsengs, and traditional aphrodisiacs such as, horny goat weed, tongkat ali, catauba, avena sativa, and elk antler velvet. Some of these are discussed below.1.) Fenugreek extracts – A 2009 double-blind study in the Journal of the International Society of Sports Nutrition, sponsored by Indian drug company, Indus Biotech, showed that a fenugreek extract tradename TESTOSURGE® , standardized for a glycosidal saponin called “grecunin” significantly raised total testosterone, as well as showing significant increases in free (bioavailable) testosterone, and over 8 weeks, significantly reduced body fat %.
2.) Tribulus terrestris – Though one Bulgarian study indicated Tribulus will increase testosterone by increasing its upstream hormone luteinizing hormone (LH), and some animal studies have indicated that it will heighten libido and sexual activity, overall the science for Tribulus is weak. Yet, some people swear by Tribulus (even including some health professionals).
3.) Maca – Though studies on Maca have failed to show any effects on hormone status,, it still has been demonstrated in human trials to increase libido, (apparently not by increasing testosterone.) Maca is reputed to be an “overall hormone balancer”, and given the number of people we have had get good results with it for libido, menopause, PMS, energy, sleeping better, etc., etc. this is a good example of where real world results speak more loudly than a lack of studies. For a surprisingly extensive review see this link at (of all places) Drugs.com See also:
4.) Panax Ginseng – Though small amounts of testosterone are produced by the adrenals, and ginseng is well validated to support the adrenals and combat stress, it is perhaps best known for its reputed aphrodisiac properties. Yet, its mechanisms of action appear to be more related to production of nitric oxide as part of the sexual response, then to raising testosterone levels. Here again, it is important to realize that boosting libido does not necessarily equate to higher testosterone. 
As mentioned, two other strategies that can be helpful in raising testosterone status include reducing its conversion to estrogen by inhibiting the activity of the aromatase enzyme, and also to reduce Sex Hormone Binding Globulin, (SHBG), the protein which binds testosterone in the blood, yielding it unavailable. A natural aromatase inhibitor is the bioflavonoid quercetin. and a natural SHBG inhibitor is nettle root. 
What else can you do?
Also helpful for acheiving desirable testosterone status is to use Saw Palmetto Extract, which is well validated for reducing the conversion of testosterone to its “darker” cousin called dihydrotestosterone (DHT), implicated in balding and prostate enlargement.
And don’t forget basic lifestyle strategies like exercise, getting enough sleep, avoiding excessive amounts of alcohol (particularly beer, because of the estrogen-promoting Hops it contains), and ensuring adequate levels of vitamin D, which can all be very important for maintaining healthy testosterone levels. A 2010 study showed that men with adequate vitamin D levels had significantly higher testosterone levels than those who were deficient. It also showed that called Sex Hormone Binding Globulin, was significantly lower in the vitamin D-sufficient group.
Related Vitamin D article: Is Vitamin D Right For You?
In conclusion, if your testosterone is low, or you are middle-aged and just simply not feeling very excited about life, chances are excellent you will get some very beneficial mental and physical effects from raising it… but your choices are definitely not limited to hormone replacement.
Disclaimer: The above information is provided for informational purposes only and is not intended to replace the advice of your physician.