Keep drinking lots of water. This is good for a number of reasons one of which is it helps keep the skin elastic. I do not know how fast you can "shrink" your skin. I would guess that with a rapid weight loss it will take time for your skin to catch up. The human body is an incredible machine though and can adapt in amazing ways. Keep up with the exercise as well because as you tighten your muscles they will in turn pull the skin back.