Diet is the main thing to get you that six pac. You might try to put your legs up on a chair and do crunches. Don't put your hands behind your head, leave them loose in fron of you. You can also lay on a table with your legs hanging off the edge. Have your butt on your hands and do leg lifts. This usually will take the pressure off your back and is great for the lower abs.
The Fool say in his heart "There is no God"