there are limited ways to tighten loose skin. one is time - loose skin can tighten with time - but i'm talking years. this depends on many factors such as age, amount of weight lost and how long you carried the weight for. two is to increase the amount of muscle you have on your body because the muscle will help pull the skin closer to your body. three - is surgery.
going back to number 2 - gaining muscle. it is quite rare for someone who has lost lots of weight and gone right down to very lean and muscular to have excess loose skin. it seems that the solution to loose skin is actually to lose more weight. get rid of all that fat underneath the loose skin and increase the muscles underneath. that, in my opinion is the best way to tighten skin.
edit: for example, you don't see any loose sagging skin on this woman:
i'm not suggesting you go quite as lean as that, but certainly lowering bodyfat and increasing muscles seems to work wonders.
