ok, if skin has been stretched and loses it's elasticity--it may not go back.
however, you can do what you can. Exercise.
here's a website with some good descriptions for you:
http://searchwarp.com/swa88879.htm
You do not get more muscle fibers. Instead when you work them they plump up. This plumping action causes your skin to fit better. Kind of like filling the skin from the inside out.
drink lots of water. Take your vitamins. Vitamins A, C, E are essential for skin elasticity. I don't believe in wasting money on creams and potions at the store--however I do believe in making your own. Cod liver oil is very high in vitamin A. Vitamin E capsules are easy to come by. You could also use some lotion to help with the "fish" smell. I like coconut oil myself.
take the cod liver oil/fish oil, vitamin E, and coconut oil/shea butter and rub your problem areas..your skin will eat this delicious concoction right up. Most of us never get enough essential fatty acids.
Make sure you are getting plenty of potassium. Potassium keeps arteries and tissue elastic...so it makes sense potassium is essential to tissue elasticity which is what you want. Yogurts can be high in potassium and easy to spread. Just read the label. Yogurt is also great for your face.
I totally believe in slathering food, yogurt, oils, all over your skin for the health benefits.
Here is something you probably do not know. diet drinks that use fake sugar actually contain a chemical very similar to formaldehyde. Formaldehyde is used by morticians to make your skin less elastic--to harden you up--preserve you. IMHO, if you are intaking alot of that junk--your skin, tissue and arteries will also do the same.