Yeah, I used to do 200crunches a everynight. Situps don't do anything, they just make the muscles by your pubes stronger. You wanna do crunches, these are like half-sit ups per-se. You do the same thing as a situp except you ben your knees so that your feet are touching the ground and each knee is about 6inches apart. Then you lift yourself halfway up and then control yourself on the way down.
It's not the number of 'em you do that counts, it's your form. As soon as I get a job I'm going to join kickboxing and get my sixpack, maybe more back