Keep your poses relaxed. Note the senior portrait on the left in the photo above. In this photo, his pose is forced and looks unnatural. Compare the pose to the head shot on the right.
Put your best face forward in your poses. Keep the focus of the head shot on your face. In the senior portrait, the focus is on the suit and awkward pose. In the right, the face is the focus and the smile looks natural.
Pose in a manner that will bring out your character. Ordinarily, crossing your arms can make you look guarded so may want to avoid it, but if done naturally it can give a more relaxed feel.
Lock eyes with the camera when posing. You will want your eyes to be the focus of the photo because it will bring all attention to your face.
If you are self-conscious about certain parts of your body, consider posing to accentuate points you do like about yourself while taking attention off the points you don't like. For example, if you don't like your double chin, take your photo with your chin pointed up to elongate the neck.
Experiment with different poses to find the one that suits you best. A good photographer should guide you in this process, but come to the shoot ready and willing to try different things.
Look comfortable in your poses and your confidence will come through in your head shot.