start and stop technique. jerk it til you're about to blow, then stop. let it go mostly limp again, then start jerking again, repeat.
OR, go buy yourself a pocket pussy (the Fleshlight is pretty popular) and use it 2-3 times daily and you'll start holding out longer with that. your stamina with a chick will get better because you'll be concentrating on a lot more than just jerking yourself off.