Basic Head Posing Rules
Keep your Tongue on the Roof of your Mouth
Odd as this rule sounds, keeping your tongue on the roof of your mouth when taking a photo slims your face.
Look to the Light
This is a trick for models doing studio photography: if you are doing a pose that involves looking off to one side, look directly into the strobe. This will brighten your eyes and produce the highlights that make your eyes most attractive.
As a side note, one thing that a photographer tries to avoid is having too much of the whites of the eyes being shown. They will attempt to guide your eyes so that a bit of white is visible on both sides of your iris. To help them with this, if you turn your head to look away from the camera for a shot, the eyes should go no farther to the side than the strobe. If it does, the highlight will be on the wrong side of the iris and make the eyes appear to white.
Pose to the Light
This is particularly important where the photographer is using shadow to define you. If you think of yourself as posing toward the light (face and body "open" to the main source of light) then your face and body will be well-lit and not in shadow.
Tilt the Head
A slight tilt to the head will give your images more character. Play with different head tilts to see what works.
Point the Chin
Many of us have a bit of extra skin around the neck so some head positions will produce the dreaded "double-chin" or "turtle-neck". Practice moving your head around in the mirror to get a feel for how to position your head without creating a neck fold.
A tip: extending the chin will straighten the neck. Try sticking your chin out slightly if you find the pose is creating neck folds.
Vary your Expressions
Build a repertoire of facial expressions. If you've held the same expression the last few shots, change it unless the photographer says otherwise. You could, for example, show a big smile, a small smile, a cute frown, a bratty pout, a scowl...
A trick is to tell yourself a little story, playing out the characters in your head.
Vary Where you Look
Another thing to watch is how often you are looking in the same direction. Looking straight into the camera tells one thing, looking up says another, looking yet another. Even when looking straight at the camera you can vary how you do it. For example, bringing the chin down to angle the face slightly down with the eyes directed at the camera can give a sexy, sultry look. Turn the head to one side and it becomes an inviting glance, looking down with a smile becomes a shy look.