Watch put for the sit ups. Make sure you are drawing your stomach muscles in(down to the floor) as you do them. If you let them round out, that is what you will be training them to do. Even on crunches you need to be aware of this. Check out some pilates info to get the whole picture. Basically, engage the abs by drawing them in like a corset around your spine. It works all those core muscles, not just the abs.