Note the indented triangle formed between the sternocleidomastoid, sweeping up behind the ear, and the trapezius, behind the neck. Observe the depth and angle of the jawbone in relation to the ear. Look at the angle of the throat and chin.
The planes of bone and muscle aren't flat, nor are changes of plane always sharp: sometimes they are so gradual that it is hard to tell where they happen. In a strong drawing, this change of plane will often be articulated with a subtle change of tone or use of implied line. It needs to make sense, reflecting the anatomy of the model, and not some 'classical' rule or guess. So think about the underlying anatomy as you draw, and closely observe your individual model.
