Research has shown that when you smile, your brain assumes you must be happy1.
A good way to start the day, prepare for a doctors visit, or a social event (especially if you tend towards depression) is to make it a habit to practice everyday for a few minutes, eventually, it will become more natural and more of a reflex action to help release stress.
Practice the following simple meditation (might help to close your eyes) for a few minutes each day:
While inhaling, smile
While exhaling, relax deeply
You can do different smiles: a subtle hint of a smile, exaggerated smile, big and soulful, eyes crinkling, mouth open or closed, or just mentally imagine smiling.
You can practice this exercise for longer periods if desired, but if you find yourself not consistently doing the exercise because you can’t spare five minutes, go back to a one-minute meditation. The cumulative value of doing this exercise over a long period of time is more important than how long you spend each morning.
1 Strack, F., Martin, L.L. and Stepper, S. (1988) Inhibiting and facilitating conditions of the human smile: A nonobstrusive test of the facial feedback hypothesis. J. Pers. Soc. Psychol. 54: 768-777