Don't stand under unnatural light
Unless you're looking to land an audition for the latest Insidious flick, stay out of any direct overhead lighting. It'll cast creepy shadows and create a generally unflattering look. If possible, move towards any semblance of natural light, or stand far enough behind the artificial light to create a balanced glow.
Take a deep breath to relax
It may sound a little hippy dippy, but taking a few moments before the shutter goes off to calm yourself will help you look more at ease and natural. Photojojo advises closing your eyes, taking a deep breath, and waiting for the photographer to count to three, then opening and breathing out just before the shutter clicks.