My dentist told me to do the TMJ exercise to stop my jaw clicking out - it helps with the relaxation too as it aligns it correctly - I find that I tend to pull it back into my neck.
It is basically standing in front of a mirror so that you can see if you are opening your mouth straight. You then put the tip on your tongue where the hard palate ends and open and close your mouth gently.
There is lots of info on the internet. This one gives you a bit more exercise and some info on posture and muscles:
http://www.rlbuht.nhs.uk/OurHospitals/Documents/TMJ%20Exercises.pdfI need new varifocals as I find I am tipping my head back to use them and putting my body into a posture where the jaw clenching happens - another thing worth considering.
Muscle tension has been a big problem for me. Every night before I sleep, I go over my whole body doing all the relaxation and breathing that I have learned in yoga classes over the years.