Place the head in space
Draw a simple head mass and mark its centre line. Check the angle of the brow and base of the nose against that tilt. A convincing structure matters more than perfectly rendered eyes on a misaligned face.
Measure relationships
Compare distances between features using the subject in front of you. Treat general proportion rules as starting questions, not measurements that every face must obey.
Use fewer values
Separate light, middle tone, and shadow into clear groups. Reserve your strongest marks for the features that organise the expression. Excess detail can make a short study less coherent.