There is no magic cure to prevent sagging skin. Sagging can happen for a multitude of reasons.
Skin can sag because the persons skin is so stretched out. There is only so much skin can take before it is impossible to have it snap back into place.
Sagging can happen due to rapid weight loss. It is best to gradually lose weight not only to instill a manageable lifestyle change but to give the body time to heal and shift back in place. Weight training helps.
Skin can also sag because lack of hydration. The skin needs hydration to keep healthy. If you don't drink enough fluids there is a higher chance of sag.
The combination of staying hydrated and gradually losing weight can prevent sagging. But there is still no magical way. If the person is huge, there is likely going to be some sag.
Searching for a perfect way to not have skin sag is like expecting a magic pill to make one skinny overnight. There is no such thing.