Vary your materials
Creatures are often unclothed, in which case material variation becomes crucial. It can be a great tool for creating interest while keeping your shapes simple and iconic. Try to be as accurate as possible when indicating a material: how does it behave under certain sighting scenarios? A good way to get a grasp on this is to do studies from photos; you can also learn a lot by observing how reallife materials react, or use a 3D program to play around with surface parameters. Also check out Scott Robertson’s book How to Render for excellent information on this.
Metal flake paint