1) Try to invoke a natural smile. Know the difference between your subject painting on a smile for the camera and an actual smile. Learn a couple of jokes to tell your model, and be ready to capture a real moment! Make yourself laugh!
2) Cross your legs. You'll look leaner, and more polished. Don't squat. Especially if you're a woman. It's not flattering. If you cannot cross your legs at the knees, do so at the ankle.
3) Posture! Pretend you're squeezing an orange between your shoulder blades to give you perfect posture in an instant!
4) Avoid a double chin by moving it forward and slightly down. It may feel uncomfortable, but it will look great. Practice this in the mirror to ensure the right technique. "Feel" how you want it to look so you can quickly and easily recreate that pose in front of the camera without the assistance of a mirror.
5) Keep things natural. If you normally don't sit outside with your arms above your head then don't do that for the camera. Overly dramatic poses look more glamour or pin-up than portrait or fashion.
6) Confidence! If you don't have much or you're just starting out... Fake it until you feel it! Modeling and posing is acting. Create a character for yourself to become in front of the camera to help you build confidence until you naturally command the lens.
As always if you've got any tips or tricks of your own please share them! Happy shooting!