If you are referring to 'losse skin' after a high weight loss then a combination of healthy eating and exercise will help improve elasticity of your skin.
The human integumentary system (skin) is not a passive layer of tissue that remains stretched out like an empty plastic bag after losing large amounts of bodyweight. Rather, it is a living organ, actively adapting to the body's internal and external environments.
So a healthy regime is the key, and drink plenty of water.
If you are referring to loss of elasticity due to old age, then ditto but maybe with supplmentary surface treatments such as Boots No.7 Protect & Perfect.