THIS LINK summarizes research with yohimbine, the active chemical in the herbal product yohimbe. You can get a presciption for yohimbine from your doctor or buy the herb yohimbe on-line or at the drug store or vitamin shop.
The conclusion of a review of studies is that this drug is not very effective. It may help some people some time. My opinion is that you can use it to mildly enhance the effect when combined with Viagra or Cialis, but that by itself it doesn’t do much. I could be wrong but that has been my experience – please feel free to post your opinion.