clean up the lineart quite a bit (jagged lines != good), up the contrast, eliminate some shades (way more than is necessary), and make sure that you're sure where your light source is when you're shading. It's lower lip is shaded when to me it looks like it's sticking out above the upper lip, and should therefore have a minor highlight if anything. I also suggest toning down the dithering and perhaps adding some AA, especially on the lip. Also try using less saturated colors and try to start grasping the concept of color conservation; if you can get the right shades of blues and greens, you could probably use some skin tones as colors on the shirt, or vice versa.
But most of all, araknidkid: keep at it.
