I do regular crunches, twist
crunches, reverse
crunches, and some other crazy things to tone my
abs. BUT I also do cardio! Walk more than 30 minutes a day, jog, ride a bike, get an exercise video--- anything to get your heartrate up.
If you don't have the means to do any of that, jumprope and jumping jacks, jog in place. There is so much you can do!
And as StarryNight said- if you don't do the crunches correctly you won't see results. I believe situps are TOO harsh on your back.