It really depends where on your body and how much sagging skin you are talking about.
After having children my stomach skin sags big time. I tried everything including exercise and decided to mention it to the Doctor one day who infromed me that the only way to get rid of it was surgery. A tummy tuck as they call it. I saw a plastic surgeon who said that no amount of exercise would remove the sagging skin. It had to be surgically removed. I decided against such a big surgery but obviously many people decide thats the road for them.