I find a really good way to crack my back is this:

Lie down on the floor on your back, with your hands behind your head.
Stretch out and then relax your back.
Now, keeping your hands behind your head, you want to do the same sort of motion as a sit-up, but do it by trying to lift your head up with your hands.

If you did it right you should get a satisfying series of cracks right the way down your back.