Semen is literally 70 percent water, so if you drink the allotted amount of water that you're supposed to daily (8 8oz glasses) then you can increase the amount a bit. The key thing is just to stay hydrated, and stay away from juices, sodas, etc because they dehydrate you and will likely make you shoot less. But, the key thing to remember is that all guys shoot different amounts of semen upon ejaculation. Some guy can shoot like a porn star, and another guy will just dribble a few drops.
Try not masturbating for two or three days. This lets it build up and when you finally masturbate! You will explode, I promise. I know from experience!!!