Well, you've got a good solid base for shading.  One thing that helped me a lot when starting out is just to keep picking away at a single piece trying different shading techniques and seeing what works.  Try using soft interior or exterior lines, dithering, hatching and anything else that strikes your fancy.  You might end up with some crappy attempts, but it's good practice.

Take a look at how some of the great pixeller's shade on the main gallery site and try to emulate what they are doing with your little cell phone... http://www.pixeljoint.com/pixels/new_icons.asp?ob=rating 

Dithering - A method of simulating many colours or shades of grey with only a few. Look at the images in the gallery to see examples. 

Hatching - To shade by drawing or etching fine parallel or crossed lines on.