Not bad. You might want to duplicate the sketch layer and set it lightly over the other layers on low opacity so you can see the anatomy you paid attention to. Still, excellent shading for an early project.

Also, a cool way to shade is grab a soft brush, scale it fairly large and set opacity to 40%. Then, pic the color you would like to shade, go into the color picker menu and dial up the black level a little. Then, paint over the area you want to shade. Painting over that same area will darken it further, set on 40% you have about three levels of shade. Then, you can smudge it smooth and it's fairly easy and pain-free.

Hey, thanks for the pointers.