I used to have this pretty bad. I eventually noticed that I was mildly clenching my jaw dozens of times per day. To break the habit, I used "The Quieting Reflex" by Charles Stroebel. It worked great.

I sometimes also get problems from sleeping on my side. Seems like that compresses my jaw and neck. Often I have to make sure I sleep on my back to avoid this.

Getting my neck stretched out with inversion and traction also seems to reduce my tension there, presumably by taking pressure off nerves coming out of the vertebrae (or rather in between). I got the idea from a study by physical therapists that showed that TOS patients who received neck traction in addition to regular treatment showed more signs of improvement.

HTH