Real skin isn't perfectly smooth; it has pores, wrinkles, and imperfections.
The corners of the mouth generally line up with the centers of the pupils. Focus on Value, Not Lines Secrets to Drawing Realistic Faces
The top lip is usually darker because it tilts away from overhead light. Real skin isn't perfectly smooth; it has pores,
Use a kneaded eraser to "pull" light back out of dark areas for a glow. Real skin isn't perfectly smooth
Ensure you have a true black and a bright white highlight.