First start off with really small stuff, like sticks on the ground. This will get your technique down. You really need to "explode" upward with your legs. I think that I may lift up on the bar a fraction of a second before I jump up, but its pretty much without thought now, so it's a little hard to describe. Jump with your legs straight up (clipless pedals really help) and then bring the bike up to you by bending your knees. On your way down, bring the bike down to extend your legs a little and help absorb the impact. As you begin to jump higher things, start trying to jump over empty cardboard boxes. Usually even if you don't make it, you're fine (unlike high curbs). A 3 foot hroizontal stick laid across some boxes works the best.