Ok, bear with me, here is another cell shading practice piece. Biohaz_Daddy suggested I try the selection tool rather than the brush so I figured I'd give it a shot. I used my older Blue Devil pic as a test piece. I would never have thought that using the selection tools with a mouse would have been easier than the brush tool but sure enough it was. I didn't spend as much time on this one as I would have liked. No excuse for hurrying except that I needed to go to bed and I didn't want to have to wait to see what it would look like. I also tried doing highlights as well as shadows. Not sure if I like that or not. Seems to give it sort of a glowy aspect.
I think in this case it's your values not technique that may be giving you trouble. The good news there is if you used layers for each level of posterization you do, it's easy to make changes.