I messed with this one for a long time, and for some reason it just wasn't working. I couldn't put my finger on it, but something about it just wasn't right. The background was originally green, and on a whim I changed it to red, and then everything clicked (well, for me anyway!).
Moral of the story: Unless you're sculpting Mt. Rushmore, don't be afraid to push yourself a little farther and try something else. You can always click "undo" it if it doesn't work.
Drawn in Photoshop on the graphic tablet.
Here's the original sketch.