This one has given me some of the best orgasms I've ever had. It's not complicated, but it takes a surprising amount of willpower and concentration.

Masturbate any way you like, though it's probably best if you're in a pretty relaxed and stable position. Continue until you're fully erect, but haven't gone so far that you're ready to come. As you continue to masturbate, squeeze the same muscles that you would use as you were going to pee. (In case this you're worried, let me reassure you that you can't usually pee when you're erect.) At first you'll find that this pressure actually makes your penis relax a little or go a little limp--it may even feel like you're fighting against the orgasm. Don't worry about that and just keep masturbating while holding the pressure. This is where the concentration comes in. It's hard to hold up this pressure for long, and you may find yourself relaxing or even trying to pull the muscles in the opposite direction (which will stiffen your penis again). Whatever you do, keep up the internal pressure like you're trying to pee. If you relax, push down again. Sooner or later as you'll start to get this tingling sensation in the back of your skull as the orgasm approaches, and then hits with enough force to blast away all resistance.

As best I can figure, the pushing really is you resisting the orgasm, but when your body senses this it sends an extra-strong reaction to force its way through and make sure you come whether you want to or not.