Think ANGLES. Go through your favorite pictures of yourself and look for patterns. Do you like the way you look from a certain angle?
Facing the camera straight on is rarely flattering; instead, turn your head to a 3/4 position to give your features depth.
When you smile a specific way? Practice that smile whenever you find yourself looking in a mirror! (practice a few poses while you're at it!)
Think CONFIDENCE. Be proud of yourself - if you think you're ugly, you'll convey that to the camera. If you have concerns about your teeth, smile with a closed mouth. Remember, the key to a bad photo is being nervous and thinking it will be bad...
Keep your best poses in mind the next time you have your photo taken. Is there someone you admire who always looks great in pictures? Think of/be that person while the camera is looking at you!
Remember, above all, photography is a game of averages - a crapshoot. The more pictures snapped of you in sequence, the easier it will be to find at least one that passes self-muster.
And then there is your secret weapon - PHOTOSHOP - don't be afraid to use it!