Last-Second Stimulation
I discovered this technique when I moved into my new dorm room for the summer...
All you really need for this to work is a porcelain sink at about waist height. Masturbate yourself to near orgasm. When you're just about to shoot (you know: that point of no return when you can't stop the orgasm from coming) touch your balls to the cold porcelain sink. The last-second stimulation really sets me off. What's better is that, since I'm right there, I can just shoot into the sink and minimize clean up.