After watching the tutorial on layering, I decided that I should give the technique a try. So, I created transparency layers with a black background, played with the morph brush to distort the figure's geometry so it looked like she'd been in a fight, and then applied everything to a render.

Overall, the effect works pretty well. However, I had to turn the main specularity off on all the layers, otherwise it seemed to multiply on the skin. That issue was easy to solve, but I can't figure out why my character's lips have a black line around them. This makes no intuitive sense, as I've plugged the layer nodes into the transparency channel, and the background for the layers is completely black.

Any ideas?