That depends on what the underlying cause of the double chin. It could be skin, fat, and/or muscle. You're going to usually get the best results with procedures that deal with what ever is causing the excess flab. It may be a neck lift or face lift. Other times just muscle tightening and neck and jawline liposuction will do the trick. Unfortunately there's no exercise you can do to fix a double chin.
“One popular new plastic surgery technique is called lip grafting, or "fat recycling" wherein fat cells are removed from one part of your body that is too large, such as your buttocks, and injected into your lips; people will then be literally kissing ass.”