Spend 20 minutes doing 30-second "stick-and-flow" drawings.
Using the same pressure everywhere makes a drawing look flat. Use heavy pressure for deep shadows and light, ghost-like touches for subtle transitions.
Spend a session focusing only on how Yan renders hands or the tilt of the head.
While he works in various media, Yan is a master of charcoal (specifically vine and compressed charcoal).
Do you prefer working with tools?