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.

I am sure I am not alone in that it is getting more difficult to deliver 2 cups in an hour session as the years go by.

When I do take vitamin V works it wonders and getting thing working at getting things going again but makes the second cup even more difficult.

Any tips, tricks or experiences or am I just getting old