To draw the human head accurately, first become familiar with the basic proportions. Traditional rules of proportion show the face divided into six equal squares, two by three. The upper horizontal division is roughly at the 'third eye' level mid-forehead, the lower at the base of the nose. The eyes sit on the horizontal centre, the mouth on the centre of the lower third.
If you are skeptical of such simple mathematics, try it out on some models in magazines - it works! While this is an ideal which does not account for racial and individual variation, observing these basic proportions gives you a starting point to measure against.
By ensuring your basic proprtions are correct to begin with, you will avoid major re-draws at a later stage of the drawing.
To construct a well-proportioned head, follow these simple steps.
Begin with a ball.
Drop a line from mid-forehead to the chin. 'Slice off' a circle at the side of the head, and from the front of this circle, curve a line down to the chin. Complete the plane of the face with a line on the other side. Now add the jawline.
Construct the nose, indicate mouth and chin position and elongate skull slightly. (The distance from chin to crown is almost the same as from forehead to the back of the skull).
For an in-depth explanation of this method, see Ron Lemen's excellent tutorial.