I prefer laying crunches, much better on the back.
Lay outstretched on the ground, place hands underneath, shaped in a V above the butt for support. Lift your chest, pulling your knees toward your chest, keeping your shins horizontal. Always keep knees together. Will be hard the first couple fo times but once you get it you'll feel a bigger impact in your abs and less strain on your back compared to situps. I actually have an angled sit up bench I bought from Sports Authority for $50 but I never use it.
Leg raies (hanging or flat) are great for lower abs, get that lower "V" in before the summer.
