When you start to get close to ejaculation, place two to three ice cubes on your scrotum or crotch. The feeling is incredible, especially at the moment just before orgasm.
DOUBLE-BAG IT
Take two sandwich baggies. Fill one with hot water (but not so hot that you can't put your hand into it), and in the other baggie put about 5 ice cubes and a little cold water. Masturbate one-handed with the warm baggie on your scrotum — and when you feel orgasm approaching, switch baggies so the cold one is on.