One of my favorite techniques in learning to release tension in the jaw and tongue. It is as simple as relaxing in a soft, low back chair with your head extended back over the top of chair, and then sing. Arms should be down and relaxed and close your eyes. I discovered this many years ago, quite by accident. I was frustrated at how I was sounding preparing a solo for a church service. I gave up and just plopped myself down in a chair with my head back and started to sing. Wa La!! Lo and behold I realized what a completely different sound I was making. It was relaxed and beautiful. I used my abdominal muscles for support (which were already well trained for singing) but my head and neck area finally gave up the tension. I can honestly say it was a life changing moment in my singing career. I now use this trick with my students who are struggling with tension in their voice and body. They are always suprised by the change in their voice and it creates an AHA moment. It doesn't always work the first try, some people relax better than others (not always easy in front of someone) but once they understand the technique, I have them go home and try it. When they come back after a week, it is fun to hear about their discovery during that time!