Make sure that your face is directly facing the camera (no more than 30 degrees in any direction). Also, your head must take up at least 1/4 and less than 3/4 of the picture.
Do not make any expressions (neutral expression works best) and make sure that your mouth is closed (lips touching).
Lighting & Accessories
For best results, make sure that you are in a well lit environment and that you are facing the main light source (no harsh shadows on your face). Also, make sure to remove your glasses, hats, or any other objects covering your face (including tucking away hair bangs on your forehead)