Lube up the inside of your thighs. Then get yourself partially aroused and trap your penis between your thighs. Then start rhythmically moving each leg up and down so that they continue to stimulate your penis.

As a tip, I've heard that this technique can be even better if you shave your thighs first.

This technique is not for the impatient, and can take a while to learn the timing. Basically, you want to masturbate until you're just about to come, and then you have to force yourself to stop at the very last moment. If you time it right, you will be able to feel it as some semen rises through the shaft of your penis and will dribble out. Wait for a minute to calm down (if you touch yourself right away you'll come instantly, but it won't be very spectacular). If you wait for twenty or thirty seconds (personal timing may vary) and then start again, you can keep repeating the process, essentially tricking yourself into ejaculating multiple times.