It's hard to do shadows when you're working with black. You need a lot of highlighted areas and some areas that are quite black. Shading consists of highlights and shadows..I do mine out of my head, without looking at refs, so it's kinda hard to explain o_O It looks good...maybe start off with something not black so you can play around with shadow and highlights.

I don't know if you want help or not, so ignore me if you don't. http://www.tainted-sky.net/draw/Line...czek_wolf2.png See how I have dark spots, and light? It adds depth to the drawing I know I'm not the best at it, but maybe it will help a little...feel free to PM me if you want