A couple of points which I thought were good:

• Fake it till you make it. People are often photogenic because they like having their picture taken. They are therefore relaxed and happy when the camera appears. If you cannot muster up genuine love of the camera, pretend you like the camera. Imagine the camera is someone you love, a long lost friend, an old flame, your child at age three, or whatever you need to look at the camera lovingly. Try it—it really does work.
• Maintain your posture. Listen to your mother — remember how she always told you not to slouch? Good posture can dramatically improve your appearance in pictures. Sitting or standing up straight will make you look healthier and more alert and, if in a group setting, more attractive than your slouching companions. Just remember to breathe normally and relax your shoulders. Especially if you usually have bad posture, it may be difficult to stand up straight and not look stiff, so practice this in the mirror.

Theres plenty more advice so have a read..