Thanks, everyone!
MagmaPainter, I paint by layering. I usually paint from dark to light, starting in the shadows and gradually transitioning to the highlights through many layers. However, instead of starting with everything dark, I'll just apply those shades in the shadow regions. Then, as the layers get lighter and lighter, I move towards the highlights. It's sort of like painting in a narrow band that keeps moving with each new layer. This way the lightest regions end up with 3 or 4 layers instead of 20+! The sketch helps guide me on this process, but that initial version gets pretty well covered up by the end.
As for the sketching, normally I'll just base coat and then only sketch in the shadows. On this piece, because I wanted to do more with the lighting, I did a more thorough sketch. What I'm trying to do with the lighting is still pretty challenging for me, so I wanted to make sure I was headed in the right direction before I put too much work in on the piece.