Originally Posted by Tealeafy
Stand in front a mirror and experiment with your facial expressions a bit. If you want a smile, try a few different ways: a grin, a smirk, etc. Find one you like, then try to remember how it felt when you were expressing it on your face. That way, when you're getting photographed you can achieve the smile you got in front of the mirror by remembering how it felt on your face. It may sound strange, but it does work after a few tries.
This really does work. Like with many other things in life, practice makes perfect. And, when you're posing for photos, don't forget about your eyes. Make sure you look at the camera and don't squint. I used to ruin so many pictures because my eyes were doing something stupid, but I have the problem much less frequenly now. Practice practice practice!