Start slowly on sit up then start to do more and more. After that, add some weight or use those med balls the heavy ones. Crunches,leg lifts, crunches is like sit up but don't let you shoulders touch the ground, then go up do it agin leg lifts have your leg straight lift them about 6in off the ground then go to 12in in the air then back to 6in off the ground do about 20 and you will fill the burn. :d then you doing it right