I always use a chair to crack my back. I just sit down and do this. I put my right hand on the chair seat at my side, then twist to the right and grab the back of the chair and stretch as far as I can and my back will crack. I also have just had someone walk on my back for me too, that will crack it too. The best thing though is cross over your arms and bend over backwards on a chair back right where you need to pop your vertebrae back into place.
Just be careful though, you can really hurt yourself. The best thing you can do to make sure you don't hurt yourself is to see a chiropractor.