I used to have a strong jaw-clenching habit, and, as with all habits, I brought it into my yoga practice and then refined it. The more challenging the pose, the more I (literally) gritted my teeth. Aside from the tightness this creates in the jaw, think about the message that it sends to the rest of the body: “Quick, get worried, this is going to be really hard, this is awful, I have to grind my teeth to get through it,” etc.
The muscles of our face carry tremendous signaling power, and we want to use our powers for good and not for evil. So rather than using our facial expressions to dramatize our feelings about a given pose (and thereby reinforcing the feelings and making them a reality), we want to try to soften the muscles in the face, particularly those around the brow and the jaw. The way I did this was to begin my practice making sure that there was space between my upper and lower teeth and then by putting a small, gentle, smile on my face. Next, much the way we learn to automatically check the placement of our hands and feet in Adho Mukha Svanasana (Downward-Facing Dog), I trained myself to constantly check to make sure that I maintained these two elements—space between teeth, gentle smile.
The results were rapid and astonishing. What I learned to do on my yoga mat, I then carried out into the world with me. No more jaw tension, and much more pleasurable yoga practice.