It all depends on a) amount of loss, b) elasticity of skin, c) age and d) heridity.

If you lose a lot of weight, and quickly especially, skin will stay loose. Exercise may help some but most of the times the only thing one can do for excess skin is surgery.

This is my problem on my abodmen. Partially caused by my weight loss but partially due to my 2 pregnancies. I saw a plastic surgeon that said it will never get back to tight no matter how many sit-ups I do (and believe me, before my back went "out" I was doing 100+ crunches each morning with very little results).