I read, somewhere on this forum, that drinking a lot of water - whilst losing the weight - helps prevent this. Was only last week so, if you check out the previous posts, you may find it.
Suppose it depends on how much weight you have to loose, your age ect... I am sure exercise would help to an extent to tone the skin too. No doubt someone with a more scientific answer will be along soon to give you a better reply. Have a great day! Julia x