Inner Smile Meditation is a Chinese Taoist traditional meditation, though echoes of it can be seen throughout other traditions as well. Thich Naht Hanh writes often about "breathing and smiling," or the practice of "smiling at life."
"In ancient China, the Taoists taught that a constant inner smile, a smile to oneself, ensured health, happiness, and longevity. Why? Smiling to yourself is like basking in Love: you become your own best friend. Living with an inner smile is to live in harmony with yourself." -- Mantak Chia
After settling into your meditation seat, start to turn the attention inward. Bring your attention to the very center, inside your head.
Visualize, looking out from the center of your head and see the inside of the 'mask' of your face. Starting with the inside of your forehead, simply notice the broad, blank screen of your imagination. Next, bring your attention downward, witnessing the inside of your eyes. Notice the muscles around the eyes, and even the way the eyelids gently touch together. Feel your eyes as they are in their natural and relaxed state. Next, bring your attention downward, witnessing the inside of your nose. Watch and observe the ebb and flow of life as it passes through the tip of the nose. Move your attention to the mouth and lips. Notice the mouth in its relaxed state. Feel the space between the teeth, feel the tongue touching the top of the palate and the teeth, feel the lips lightly touching together.
Now, pull your focus back to the center of your head, witnessing the whole inside of the mask of your face.
Again, return your attention to your mouth, to your lips. Notice the feeling of the corners of your mouth as they are in their natural relaxed state. Now, ever so slightly raise the corners of your mouth until you feel the 'inner smile.' Even if it feels contrived or fake. Perhaps your smile isn't physically perceivable by an outside observer, you are lifting the corners of your mouth, until you feel a distinct shift in energy, a warmth and radiance of being.
Allow the corners of your mouth to remain in this uplifted position and turn your attention up to your eyes. Ever so slightly, raise the corners of your eyes, as if the eyes themselves could "smile." Feel the warmth and radiance brought about by allowing the eyes to smile. This is the feeling of the 'inner smile.'
Thich Naht Hanh, "Peace is Every Step"
Mantak Chia, "The Inner Smile"
Dan Landrum - http://chippit.tripod.com/inner_smile.html