When you lose weight how do you stop your skin from sagging and can sit-ups and ab crunches help?

Unforntunately, no. Skin elasticity is a genetic trait. People whose skin that can stretch without tearing (stretch marks) has a higher level of a substance called elastin, which allows the skin to stetch. Without sufficient quantities of this substance the onty way to get rid of the excess skin is with plastic surgery. Exercise will tome the muscle in the area, but you also cannot lose weight in one area alone by exercising that region.