Unfortunately, it is sometimes just the way the face is structured and has nothing to do with excercise, excess weight, etc. Quite honestly if it's not a general weight problem then surgery may well be the only option.
As mentioned before - it's usually due to a layer of fat which only weight loss or liposuction can get rid of. But you can tighten up the muscles and lift it a little with facial exercises. Another one to try is: look up to the ceiling, keep your mouth shut and touch your tongue on the top middle of the roof of your mouth. Do this several times every day - if you put your hand under your chin you can feel the muscles tighten up.