Think I heard once that to prevent the loose skin when you have a lot of weight to lose you kind of have to try and not lose more than 3lbs per week as doing it that way gives the skin a chance to recover and change with your body.

Everyone is different though and some people can lose a lot quickly with no ill skin side effects.

Anyway for those that do have the loose skin after losing all the weight there is the option of surgery to remove the excess skin but of course you need to be on pain pills for a while after that.