Well my honest opinion is that you should probably make lots of simple shapes and then practice shading on them, and always keep a light source in mind.
I don't think you've got shading completely under control, but it's getting there
I really do recommend this link
http://hippie.nu/~unicorn/tut/xhtml/#basics-lightshadow
The rest of the stuff is pretty neat too(except the furry part... unless you're into that).
Shading is hard though x_x I cant do it either. I suck at drawing but I try lol.