Regular crunches on the floor, leg lifts, and like dave said dont do them on a hardwood floor. When I do crunches at home I do them on the carpet in my bedroom, and I put my feet under my dresser, works well and Ive never hurt my back from it. You can also do Swiss ball crunches 0311 convinced me to do them,they work great, and of course if your laying down on a huge ball filled with air its not going to hurt. If you do them just make sure you dont fall off the ball in front of alot of girls (like me yesturday)
Anuj's idea of upsidedown sit ups sounds like alot of fun though, and would definatly not hurt your back...unless you fall down. I remeber reading something like this in Arnold's encyclopedia, that him and franco would do things like this after squats to relieve the tension on their spines as well. I'm sure it feels great on your back just to hang upside down period
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