1. Take a deep breath and let yourself relax. Taking pictures can be stressful, but stress and negativity leave you tense and anxious. Listen to music, sip a bottle of water or give yourself a few quiet minutes to meditate or daydream before a shoot.
2. Don't force yourself to hold your smile for too long -- you'll end up looking mechanical and unnatural. If the photographer counts down, smile on 1. If there's a pause between pics, let your face relax and then smile again just before the next photograph.
3. Think of something funny or happy before you smile -- it'll help you smile more naturally. Laughing a little helps your smile look natural, too.