poop said exactly what I wanted to say. The depth of the shadows on the head doesn't look to be the same as the remainder of the body.
Also, for variation that I spoke of you can try this:
In Photoshop (assuming this is what you're using) create a new layer and pick a few colours to use: harsh red, harsh blue and a harsh green. Nice and bright variations of those 3 colours and start putting them in places you think could use some variation. Do it with a softbrush at 100% opacity and do this on its own layer. Once you've got the layout done the way you want change the layer to multiply or screen and change the opacity of the layer until the changes are subtle.
It might not seem like much but it'll help.
I did a quick run of what I'm talking about for you.
This is what it looks like before I set the layer properties:
And this is what it looks like with the green layer set to lighten @ 17% and the red layer set to mutliplye at about 11%:
Hope this helps.