@croydon - Unfortunately it just happens sometimes when someone loses a lot of weight. Particularly when they lose it quickly and when they are over a certain age.
You can try rubbing skin creams with vitamin E into the skin, but I've heard they don't do all that much.
Another technique I've heard, which works for some people is to get your weight down as far as you can safely go, as the body might get the hint and start tightening the skin.
But that doesn't work for everyone either. Really, the only guaranteed method of getting rid of excess skin is to remove it with surgery. Whether you think it's worth doing that, and whether you can afford it is up to you.