Abstain for as long as possible (bare minimum of a day or two), edge your orgasm for at least an hour and when you finally DO let yourself cum, contract your PC muscle* in time with your spurts. This'll take a lot of exercising and practice to get right, but trust me. . .you'll shoot like a porn star.
These are called "Kegel exercises". It's the same muscle you use to stop your flow of urine. It starts out very weak (you won't be able to do it longer than a few seconds), but after a few months of stopping-and-starting your flow hands-free, it'll develop some tone.