Some research there will turn up a lot of stuff, but here's an executive summary:

Vitamin V does make it harder to come, especially the second time.

Age is the primary culprit.

The best ways to avoiding succumbing are plenty of rest, good diet including lots of water, and plenty of rest.

Also, cut down on booze and drugs.

If love is your drug, leave the others alone.

A really good lover also does the trick for me.