Open your mouth as wide as you can.
Hold this position for about 10 seconds, then relax your mouth, and repeat the exercise a few more times.
For faster results, add resistance by placing a tennis ball underneath your chin, while testing your jaw span.
Sleeping without a pillow is said to help.
Also, you can massage the area daily with a cream. Start from the chin and go downwards using both hands, one hand follows the other.