Stay hydrated. Try to hold back as long as possible. Allow a few days to pass between ejaculations. Try doing some kegels to strengthen your ejaculatory muscles (google 'kegels exercise' if you don't know what I'm talking about there. Make sure your nutrition isn't vitamin deficient. Not sure what position you're in when you cum, but try not to point straight up or gravity is working against you. Last but not least, you could also take a volumizer if it's really important to you. (Trying to impress the guys in a circle jerk, are we?
) I tried one and it did work as advertised. Didn't make me more potent or horny really, just made me shoot more when I came.
In general, the more your body has to shoot, the more that comes out, and at greater speeds. Most dribblers can shoot, if only once or twice, by just holding back for a few days. Wisdom from someone who does shoot: Point that thing AWAY from your eyes.