Here's what u do:
a) practice getting just to the point of cumming, then stopping. Make the girl stop too. Tell her to stop and grab her. Then after a few seconds you can start again, but ever time you feel like ur going to cum u stop again. The first few (ok, the first 100) times, you'll probably go too far and cum anyways, but eventually you'll get it.
b) if u get good at this, u can work up to being multi-orgasmic. For that, you let yourself cum just slightly, then stop, then u can start again. That one is tricky, but if ur really horny and practice, it can be done.
good luck