There are several simple exercises you can perform to achieve that goal. Please read on.
Look straight ahead and crane your chin upward. Push your jaw forward, and bring your mouth into a kissing position. Hold the pose for 30 seconds.
Place your chin on your folded hands just like the girl in the image. Gently apply pressure: push up with your hands, and push down with your chin.
Looking straight ahead, tilt your head back a little. Now move your jaw as if you are chewing a pretend stick of gum.