It's common for many of us when first starting our practice to clench our mouths and tighten our faces as we try to learn the postures. Aggressive or competitive personalities especially tend to have this habit. We can also carry a lot of tension in our jaw muscles from anxiety, stress, and anger which can result in neck and shoulder pain, headaches, tooth pain, damaged tooth enamel, and TMJ syndrome.
Relaxation is hard work, but developing a keen awareness is the first step to overcoming the harmful habit of unconscious clenching of the jaw. Unless you are chewing food, your teeth should never be touching. Try to be aware of this especially during your practice. Keep your lower jaw relaxed as if it was hanging in a sling. Relax your tongue and remove it from the roof of your mouth. Keeping your lips closed, relax your lip muscles and stop making those funny strained faces in the mirror.
You might be surprised that as you focus on relaxing your mouth and jaw, the rest of your body and mind will follow.