Nothing super groundbreaking this time, but I did want to point out something I do in most of my finished work that isn't in my commissions right now.
The difference between an object in the light and an object in the shade isn't entirely as simple as adding a little white or black to its colors. Areas of the subject that are in the shade have cooler (bluer) colors, and areas of the subject that are in direct light have warmer (oranger) colors. It's a super simple effect, but it makes a huge difference in terms of perceived depth.
I also color the linework accordingly. It looks a bit weird on its own, but it blends in much better in a finished piece.