I borrowed this from a great portrait and wedding photographer David Terry.
Excellent advice from one of my photographer friends (Doug Havens):
When someone points a camera at you, you might be trying too hard to "have your picture taken". Can I offer some tips?
1- Think about things you love. REALLY love. Let that emotion build up
inside of you. Beauty is not what's on the outside of you, it's what's
on the inside shining through. Happy thoughts of love will make better
photos.
2- Relax. If you are
thinking about your pose you're not following #1, shake your head and
your arms and repose and think of happy loving things and tell the
over-directing photographer to shut up. :)
3- Be confident. Know that you are the most amazing person and know the camera will record that - but relax! :)
4- Smiling isn't always the best photo. Looking straight into camera
isn't always the best photo. You are an individual, and the best way to
be photographed will vary from person to person
5- If you're a woman, turn one shoulder slightly to camera. It will slim you.
6- Realize that most of the burden of producing a great photo is on the
photographer. We have a LOT more to worry about on our side of the
camera. You just be your fabulous, happy, loving self and let me do the
rest.
There are shoots where it takes 200 photos to get my
subject to relax, and when they finally do, it is just magic. You can do
this! I hope this helps.