Basic rules for posing
If you look at red carpet poses you will notice that the stances are almost the same. For girls it's one foot slightly past the other foot pointing forward towards the camera. Why? Because these certain poses automatically align your body creating "good lines." If you are slightly over weight, believe it or not, you will want to sit and be shot a little from an above angle or you'll want to stick your neck out. It hides, it puts your face more prominent and it prevents the ol' double chin.
Guys, one for one I have seen men hold their heads high and pop out their chests. It takes away from your jaw line, it looks uncomfortable and rarely ever works in a shot. On the flip side to the head held high, angling your chin down can be very nice; but if you go too far you will look like a serial killer. The truth is that your photographer will see these lines and these poses and will be able to spot what works for you. You are an actor not a model! If you have attention on posing, spend a little time in the mirror. Know that with the mirror you are seeing yourself head on, but still play around and get an idea of your head and body and arches and slouchings and so forth and work out little kinks. But don't obsess on this please! More import than "posing" is to communicate with your photographer. The photographer will help you. If you have too much attention on this then you will end up freezing or focusing on the wrong things.
These are just really simple simple simple basics. You are not a model. You should pick a comfortable experienced photographer so that you do not need to stress out about these things. I only added this section because so many people ask.
NOTE TO PROFESSIONAL MODELS: For the first time in perhaps quite a while this photo shoot is not about something you are selling. This photoshoot is about YOU! Focus your communication to the camera, not your poses. This is perhaps the funkiest part for a model getting headshots. Looking at the camera and communicating "This is me."