Every man's hormonal situation is different and some men want to lower their DHT (dihydrotestosterone). There can be many reasons for this including
Whatever the reason, I see concern for these issues on the Peak Testosterone Forum quite often. Compounding the problem is the fact that the big pharmaceutical inhibitors of DHT (Propecia or finasteride) are rife with side effects and so men are often reluctant to touch them. A number of experts believe in something called "Post-finasteride Syndrome" and believe it can cause permanent sexual side effects. I cover some of this in my page on The Risks of Propecia Use.
Regardless, men are often looking for more natural ways to manage the problem and, below, I include several herbal and dietary ways that may lower DHT without using a synthetic pharmaceutical solution: (Always discuss everything first with your physician if you have any medical issue or are on any medication.)
1. Saw Palmetto. This is the most well-known of the herbal 5-alpha reductase inhibitors and has many studies showing its powers. How effective is it in real life? Well, it actually has a couple of significant studies showing that it improves BPH (enlarged prostate) for example. 
2. Stinging Nettle and Pygeum. These are widley used in herbal prostate supplements and research shows them to be 5-alpha reductase inhibitors, especially when used in combination. 
CAUTION: Saw Palmetto, Stinging Nettle and Pygeum have a better track record that Propecia. However, some men on the post-finasteride boards report that these herbs caused them trouble.
3. Progesterone. Progesterone is the new men's health hormonal kid on the block and some research has shown that it can even improve erections. In fact, I have a whole page on the subject: Pregesterone and Erectile Dysfunction. Progesterone has an interesting property: it is widely reported to lower DHT in men. Is it safe for men in the long term? Dr. Mercola has stated that the cream version can accumulate in tissues in some cases.  Some say it is safe and prostate protective, but very little research comparatively has been done.
4. Riboflavin (Vitamin B2). One Japanese study found this to inhibit the 5-alpha reductase enzyme.  Unfortunately, dosage is not well-known.
5. Green Tea. Green tea has two powerful catechins, ECG and EGCG, the latter being responsible for most of the anti-aging and superfood characteristics exhibited by green tea. Black tea does have some of these compounds, but not nearly as much. (Black tea can lower your blood pressure and boost blood flow though!) But green tea has enough ECG and EGCG to where some believe your DHT could be lowered enough to notice. 
We had one forum member claim that black tea probably lowered his libido and that rodent studies showed that DHT could be lowered with black tea. I could never find the evidence for that and am skeptical, but here is what he wrote:
"I'm British and drink a lot of black tea (with milk), there was a few studies claiming black tea lowered dht in rodents by 72%, more than finasteride (hair loss drug) that reduced dht by 70%. I know people say this isn't related to humans because it was done on mice, but mice were found to to have 70% lower dht on finasteride, exactly the same as humans. I drink about 8 to 10 cups a day, I know, a bit excessive!" 
Now, while green tea may lower DHT a little, it is truly an incredible superfood and even displays anti-aging superpowers. I have a glass every day. That said, I wouldn't "go crazy."
6. Quercetin. I took quercetin for awhile when I had a high PSA. It is an anti-inflammatory and some polyphenols are also 5-alpha reductase inhibitors. There are indications that quercetin may lower DHT as well.  Is this safe long term? Again, taking megadoses of quercetin is a relatively new phenomenon - no one knows the answer for sure.
WHAT ABOUT ZINC? I have read in many places that zinc acts as an 5-alpha reductase inhibitor. However, as you'll see in my page on Zinc and DHT, there is a well-done study that shows men actually increasing their DHT with zinc supplementation. Also, you may want to scan through my Zinc Dangers page.
CAUTION: Remember that DHT is responsible for a significant part of a male's libido. So reducing it too much could have a negative impact in the bedroom.
1) Carcinogenesis Advance Access published February 12, 2004, Hung-Hsiao Lee1, Chi-Tang Ho2 and Jen-Kun Lin1,* , "Theaflavin-3,3â€™-digallate and Penta-O-galloyl-Î²-D-glucose inhibit rat liver microsomal 5Î±-reductase activity and the expression of androgen receptor in LNCaP prostate cancer cells", http://carcin.oxfordjournals.org/content/early/2004/02/12/carcin.bgh106.full.pdf
3) J Endocrinol, Jun 1 2004, 181"493-507, "Reduction of rat prostate weight by combined quercetin-finasteride treatment is associated with cell cycle deregulation"
4) The Journal of Antibiotics, 1990, 43(12):1615-1616, "Riboflavin, a testosterone 5 alpha-reductase inhibitor"
5) Am J Chin Med, 2004, 32(3):331-8, "Saw palmetto and benign prostatic hyperplasia"
6) Aging Male, 2004 Jun, 7(2):155-69, "Preventing diseases of the prostate in the elderly using hormones and nutriceuticals"