It will tighten up naturally if you exercise on the treadmill. Treadmill works. Jogging in the park or around your block is the same thing as the treadmill.

I have experienced saggy skin do to 120 pounds weightloss a few years ago.
what has helped tighten mine is exfoliating scrubs and then following the exfoliation with some elastin collagen lotion. you have to use a a exfoliating tool to massage the area you need to be tightened first you soak your skin in a warm bath for about 5 mins. then put the exfoliating scrub ( I use a purifying salt scrub )on the area you want to work on. then you take the exfoliating tool. ( I use a tool you can find most any store called a pumus stone ). you massage the area with the stone and exfoliating scrub in small circles until you see the skin starting to turn a healthy pink color, then you can move onto another area. I do this massage at night before bed then follow with lotion. and let the skin rest and heal for 3 days then do it again.well it does work but it takes time to see results I have seen my skin tighten and the stretch marks fading but if you have real bad sagging the only thing that will tighten the skin is plastic surgery.