A couple years ago I was pleasantly surprised to receive a Wacom tablet as a joint birthday present from my extended family. Most of them didn't know what it was, but I explained it was like an Etch-a-Sketch and that I was really happy!
Recently I found some great tips on www.danidraws.com. Dani Jones is a great illustrator (and teacher). Her video tutorials demonstrate digital painting techniques that really make sense to me. The key (in one sentence) is to use textured brushes to build the color with varying degrees of opacity.
The first image is the final illustration. I've removed the line art on the second image so you can see the technique a bit more clearly. To see great illustrations and tutorials visit Dani's website!