Just a little pop of feedback as everyones mentioned any advice I might've laid on! xD
So, I like the strength of the outline and I think the only thing missing is the feel of subtle and strong features which are easily achieved by some experimental shading. When doing this I suggest printing a good five or so photocopies of a piece and the just try shading them in different ways. The most important thing is not to be afraid to be heavy with shading, you can always tone up with more or tone down with an eraser.
Keep it up
Ben