I have gotten great results from using yohimbe. It keeps ya stiff for hours but is also a stimulant and keeps you wide awake for hours....not very easy to fall asleep. So in other words, it keeps ya up all night....no pun intended.
The best brand i've ever used believe it or not is GNC's brand of yohimbe. Start out with the smallest dose possible to assess your tolerance. A little goes a long way.