I recently had an interesting experience where I suddenly found that my trough testosterone levels increased by 100-200 ng/dl over previous levels, representing a 14-33% increase over all my past reads while on testosterone cypionate. I attributed this to the fact that I had gone almost entirely vegan in an effort to control my PSA. (Strict plant-based eating can lower IGF-1 and increase SHBG a little.) However, there is another much more likely explanation: quercetin. I had also started taking quercetin (along with turmeric, bromelain and IP6) in an effort to control the inflammation that they found in my prostate and, after some reading, I found that quercetin has been found to increase testosterone levels.
Quercetin, which is a powerful anti-inflammatory phytochemical compound found in red wine, onions and other plant foods, is particular good at blocking a liver enzyme (UGT2B17) that tells the kidney to urinate out testosterone. In other words, quercetin allows your testosterone to circulate a little longer.  Granted, quercetin isn't going to cure a hypogonadal male and double his testosterone. But it can help a little and it can help many men on HRT .
NOTE: This could be a significant help to men with low SHBG. Low SHBG levels cause testosterone to be rapidly cleared from the body (through the urine) and makes HRT often ineffective. Talk to your doctor, but in my opinion it is likely that quercetin could help with these type of hormone clearance issues. For more information, see my page on Causes of Low SHBG in Men.
Quercetin's impact on hormones has been in the spotlight lately, because the powers-to-be are interested in helping out low SHBG men or boost male testosterone levels: officials were concerned about possible "doping" issues in sports. Athletes are always looking for an edge and there was concern that this was being used to alter urine hormone excretion. (Sometimes I think the winners are likely just the best cheaters.) A little boost in testosterone is nice, but that's just the beginning of the good news with quercetin, including a few items that will almost for sure help you in the bedroom:
1. Prostate (PSA) Protection. Remember that high inflammation can inflate your PSA and a high PSA will take you off of HRT. In fact, this happened to me as I mentioned. My PSA went from 1.4 to 6.3 from one read to the next! (You can read about my experience here: High PSA But No Cancer.) I started taking quercetin, because a study on men found that 500 mg of quercetin (with bromelain and papain) twice daily significantly improved prostatis, which is almost always a inflammatory issue.  The prostate does not have good blood flow compared to most organs and thus has trouble clearing out toxins and bacteria. Quercetin can help with the former.
2. Lowered Blood Pressure and Increased Blood Flow. If there is one thing that will help you in the bedroom, it's increased blood flow, eh? Quercetin will lower blood pressure for many men, especially the ones that need it most, which means it's opening up your arteries and allowing greater blood flow to your extremities. Nice! I know one extremity that could always use a little extra blood flow! And one study found that systolic and diastolic blood pressure were lowered by 7 and 5 mm, respectively. See my page on Erectile Dysfunction and High Blood Pressure. Remember that, if you are have good blood pressure, quercetin will probably not help that much.
3. Brain and Dopamine Protection. Recent study work on quercetin and resveratrol suggest that these can protect you from dopamine depletion and possibly Parkinson's due to their powerful anti-inflammatory activities.  It turns out that Parkinson's Disease has a strong component of inflammation that researchers now believe is one of the key root causes of the condition.
4. Prostate Cancer Protection. Results are preliminary, but it looks like quercetin has good potential to help your prostate in the battle against prostate cancer. 
5. Cortisol Control. We're all under chronic stress and one study showed that quercetin can put the clamp on stress reacitivity, i.e. your body overproducing cortisol due to various stressors. Researchers found that rats give (admittedly very high doses) of quercetin limited their production of a hormone called CRF (corticotropin-releasing factor). 
6. Fertility. Animal studies show that quercetin should boost sperm quality and fertility. 
7. Protection of Your Testicles. Animal studies also show that quercetin has your back and can protect your testes from all sorts of dangers - everything from torsion to toxins. 
DOSAGE: What dosage should you take? Well, first of all, talk to your doc if you have a medical condition or are taking any medications, because supplements can alter medication absorption and have unintended consequences. However, in my case, my prostate biopsy was found to have no cancer or pre-cancer (PIN) but rather inflammation. I started taking 250 mg of quercetin (with 125 mg of bromelain) twice daily and later switched that to one daily. Unfortunately, I am not sure if I was taking 250 mg or 500 mg daily at the time of my increased testosterone read. The reason that I backed down in dosage was just to move the amount I was taking a little closer to what you could get through food sources.
For example, 5/8 of a cup of red onion would be 100 grams and 100 grams would supply about 32 mg of quercetin.  So, if I ate 8 red onions, I could get 250 mg of quercetin. Well, okay, that's not too likely, but the point is that trying to move it into the ballpark range for human consumption and 500 mg definitely would not do that. That said, remember that in the above study for prostatitis, which is a very common problem in men, the dosage was 500 mg 2 X daily. And the above blood pressure study was 700 mg daily.
1) Science Daily, Jan 7 2013, "Red wine could mask testosterone levels, experts warn"
3) Urology, Dec 1999, 54(6):960 963, "Quercetin in men with category III chronic prostatitis: a preliminary prospective, double-blind, placebo-controlled trial"
4) Journal of Neuroscience Research, Feb 1 2008, 86(2):403 410, "Resveratrol and quercetin, two natural polyphenols, reduce apoptotic neuronal cell death induced by neuroinflammation"
5) Carcinogenesis, 2001, 22(3): 409-414, "Quercetin inhibits the expression and function of the androgen receptor in LNCaP prostate cancer cells"
6) Molecular and Cellular Biochemistry, Jul 2006, 287(1-2):109-116, "Quercetin downregulates matrix metalloproteinases 2 and 9 proteins expression in prostate cancer cells (PC-3)"
8) Asian Journal of Andrology, Mar 2008, 10(2):249 258, "Stimulating effects of quercetin on sperm quality and reproductive organs in adult male rats"
9) Andrologia, June 2012, 44(3):164 173, "Quercetin prevents 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced testicular damage in rats"
10) Andrologia, Dec 2010, 42(6):376 383, December 2010"Protective effects of quercetin on testicular torsion/detorsion-induced ischaemia-reperfusion injury in rats"