Lie flat on your back, with your arms extended from your shoulders (think "T" shape). Take your right leg and roll over to the left, keeping your right arm down to the ground as much as possible… Repeat in the other direction.
This usually gets 3-4 of the lower vertabrae to crack for me.
I just want to rant and rave about how great this technique is - I got my first real back crack out of it. It's a great way to start the morning.
Though in addition to simply rolling your right leg over to the left (and vice versa), I'd also recommend grabbing your leg with your free arm and pulling it towards your chest as hard as you can without exerting yourself. It requires a little bit of flexibility, but I've gotten some incredibly satisfying cracks out of this.