As always, first see if there might be a physiological reason, such as an incorrect bite or a missing tooth. Sleep apnoea is another possible cause that should be checked.
If you feel that stress might be the main reason, there are several things you can try to get out of this habit.
First: Eliminate the Cause
It can sound superfluous but trying to understand what your triggers are and what you can do about them and actively reducing your stress levels is paramount.
Second: Break the Habit
Many people use acupuncture in order to relax those tense facial muscles. But you need to keep them loose as much as possible in between treatments.
You can start gently massaging your cheek, jaw and lips even stretch them carefully; find those tender and tight muscle strains. Most people grind their teeth at night, but you might find that you clench them during the day without noticing. Consciously start unclenching, put the tip of your tongue between your teeth. This position unclenches the jaw.
And of course, the usual killjoys ... : cut back of caffeine, chocolate, alcohol ..