For better shading try to think in 3D about materials behaviour (skin, hair) and guess how thing should be lighted and what will shadows look like. Don't forget about contrasts, colors mixing and overtones ;-)

I know, I just can't seem to get the hang of the shading.

But if I keep going at it I'll get better.... at least that's what my mum always told me :P

Linework is nice, other than the somewhat clunky and too-sharp linework of the hair, but the shading doesn't really give any indication of volumes.