Hi, I thought you may like this Mentalrobics exercise from Braingle.com:
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Relax the Jaw

When people are under stress [ For more on 'stress' go here: http://www.braingle.com/mind/534.html ] they often clench their teeth. This can lead to tight jaw muscles. This tightness can then spread to other muscles in the head and neck and may even contribute to back pain and tension headaches [ For more on 'tension headaches' go here: http://www.braingle.com/mind/694.html ]. The following stretching and massage exercise will help eliminate the problem at the source.

First, open your mouth as wide as it will go to get a good stretch of the jaw muscles. Do this a few times. Now, try to open your mouth again, but use your hand to give some resistance. You should feel your jaw muscles trying hard to open your mouth against the force from your hand. Maintain this pressure for at least thirty seconds and repeat a few times. This may help teach your jaw to stop clenching. Now, take your two fingers and find the small indentation at the back of the jaw muscle near the ears. Push on either side until you feel a slight pain and massage the muscle a little bit. This will help relax the tension that is already there.
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For more mind exercises: http://www.braingle.com/mind/