Dithering is pretty good, but I wouldn't use it unless I was adding a rough texture with a piece this size. It makes the butt look like it has bumps on it and that's probably not what you're going for.
Speaking of the butt, it looks a bit too square on the right side of the image (the line shouldn't go that far to the right for a leg that's forward). The silhouette of the right hip bone shouldn't curve out *quite* so much at the top. The curve under the right cheek should start just a *little* higher.
The left side looks okay. The cheek just looks beat up though. I see what you were going for, but the shadow should go along the muscle in the back of the leg (that muscle shouldn't be prominent enough to be shown though). You can keep the dip in the cheek, but it should start a little higher also. It's supposed to be round, so I'd give the lighted area of the cheeck (toward the crack) a little more *pop* if you know what I mean. Make it rounder. The big crease on the left leg simply shouldn't be there. I think you're confusing two muscles as if they were one big one. The butt is its own muscle after all.
If you're just doing backlighting on the left side of the butt, you don't need any deep crevice as it appears you have now. You could do a transition gradient from under the left cheek to the part of the leg that's going more into the camera rather than breaking it up with a darker color like you have now.
The light source seems a little inconsistent too where the butt and the back are concerned. The top of the butt should be brighter and just graduated to the darker areas. Even if you're doing SNK style shading, you would still not use dithering on smooth soft skin unless absolutely necessary.
Other than those things, not bad dude. I really like the shirt and hair so far.