Besides the supplements and techniques mentioned above, you could be surprised what effect can a prostate massage have. Little pressure or vibrations on the prostate during climax makes wonders.

I also can say, that zinc+selene, arginine and maca works for me. But most crucial is to be well hydrated. Overall health is good too.

Quite well written is "Secrets of a Porn Star Cumshot" by Steve Sizemore. It is available in the pdf - just search the internet for it.