Try not to focus too much on how you look. Do you ever really focus in your everyday life on how your face looks like at any given moment?
It's important that you avoid a double chin, therefore try to move your forehead towards the camera in order to elongate your neckt and getting rid of a double chin and wrinkles on your neck.
Make direct eye contact with the camera.
Angle your head slightly to hide asymmetrical features of your face. The left side of the face is for most people the better side.
Squint very slightly to give your expression confidence.
Unless you have naturally a great smile that comes effortlessly while being photographed, it's better not to smile at all, relax your jaw and keep your lips soft.

Check out Peter Hurley's video and hear him talk about squinting/squinching:

http://www.youtube.com/user/peterhurleyphoto