I got this tip from a tutorial by Robert Atkins.
To draw facial expressions, draw a cartoon variant first. It helps you simplify and focus on the essentials of the facial expression. Then Draw that same expression in a more realistic style, guided by the cartoon you drew.
For this exercise, find a mirror in which you can pull faces. Then draw a smiley equivalent of the expressions you make in the mirror: draw the eyes, eyebrows, mouth.
Then draw a face — it doesn’t have to be your face! — with that expression, using the smiley you drew of the facial expression as a guide.
Pull a face in a mirror, and draw the facial expression in a cartoon style. Then draw the same facial expression in a “realistic” style.
Try this also: find photo reference of faces, and draw them with a different facial expression.
The cartoon style is a simplification which helps you find the essentials of the facial expression. You can then apply these essentials to a realistic rendering.