A bit more attention to detail may be in order, as regards the shapes and angles, especially the bridge of the nose cavity, the hole under the eye socket and the upper jaw bone, which should undulate along the tops of the teeth. You also need to make it clearer that the back of the jawbone is behind the bit that extends backwards from the cheek bone. The hole in the jawbone is also too big. The outline needs to be less jagged on the top of the skull.
I would explore the differences in hard and soft edges. This is a good practice to start early when learning, I'm still having a problem with it cause I ran past it. XD
You can see where some edges, shadow and light, collide very bluntly and create a very fine straight edge which you would try and use a very hard brush with little not no opacity to create, and then you have shadows bleed out into the lighted areas which you would use an opacity brush or soft brush to create the softness for said edge.
This is pretty much a rendering study for you, so get down the detail. Make your brush small if you have to and get in there.
I would also suggest doing this is color and not white, as a way of seeing value within color rather than black and white. (you can throw a color layer over this and paint over the color for which you want to use)
Thanks guys that was great feedback, just what I was looking for.
Spent another 2 hours tidying up the lines, some of the angles, and generally rendering it better. Started zooming in and really getting in deep rather than sitting back at a zoomed out piece and saying 'done'
Here's the update. (Still avoiding colours for now, learning process - might colour it in once finished though).
[edit: still haven't gotten around to sorting the teeth section out)