Jimbo: The free form amino acid L-Arginine works fairly well and you notice results after about 3 weeks. Zinc and vitamin E also seem to support semen production. A good product that combines all of these is Male Fuel by Twinlab. It also has nutrients that foster good "male" health as well including yohimbe which makes you horny as hell.
Quite a few "libido enhancers" are supplements composed primarily of L-Arginine and sold in the male health section in health/drug stores under names like Arginmax. L-Arginine also increases the nitric oxide in your blood which enhances erections somewhat like Viagra and this is the effect that is primarily marketed in these high priced supplements. It you decide JUST to do the L-arginine, get the CHEAPEST free-form brand you can find. You need to take 3 to 3.5 grams per day on an empty stomach in order to create the semen increasing effect.