Hm, it doesn't look too bad for a bit o' flatshading, really. You've got the pose and such right, that's a great 'solid' bit of overall drawing talent.

For me for shading... I use GIMP for all my coloring bits. I more or less magic wand the area I want to color, dump color in it, go to a different layer with a multiply bit on it (with the same magic wand selection as before), and draw in the new color manually, using a brush with not-quite-full hardness. This makes a nice hazy edge. Then I just draw in the rest of it (the selection stops it from drawing all over the picture), and I wind up with a somewhat nifty cartoony shading thing.

But, that's just me. :-)