You can always try doing kegel exercises to strengthen your pubbocoxygennus (PC) muscle. I don't know any sites off the top of my head, but you can google for some exercise routines. The PC muscle is the muscle that pumps when you're ejaculating, and if you're able to strengthen and control it through these exercises, you can voluntarily contract the muscle at the point of orgasm. This will cause you to orgasm without ejaculating. When that happens, you'll still get the feeling of euphoria but your penis will stay erect and you can just keep on going until she's gotten enough.
On top of extending your lasting ability, doing kegel exercises is supposed to reduce the risk of prostate cancer, so all the more reason to try.
