Make the "mmm" sound while relaxing your lower jaw and tongue. Do not let your teeth connect while doing this. Add resistance to the movement by making a fist with your hand and pushing it against your jaw while opening it.
Move your jaw up and down while relaxing your neck and shoulders. Open your mouth as wide as you can and then close it, never allowing your teeth to touch.
Push your jaw from side to side, holding the sideward stretch for a count of three. Add resistance to this exercise by gently pushing on your jaw with your thumb in the direction of the movement.
Tilt your head back, but keep your shoulders relaxed. While in this position, push your tongue against the roof of your mouth and hold it for a count of three. Slowly lower your head while your tongue is still pushed against the roof of your mouth, and release once you've returned to the relaxed position.
Chew gum. The regular chewing movement that results from chewing gum can greatly increase the strength in your jaw and build muscle.