An exercise I'm doing is giving my lower-jaw some resistance with my hands while opening and closing it, or holding my chin firmly while i make circular chewing motions.
Also good is to exercise your facial muscles and cheek muscles by contracting one side of the face tightly, stretching the other, -holding it- and then reversing.
Another one is to tighten your lips and frontal muscles tightly, then fill your mouth with a bit of air and apply pressure from the tongue and cheeks, keeping your lips tight so no air can get out.
Note: I'm not the inventor of these, they are from the Charles Atlas Bodybuilding Course. Cool exercises though.