a suggestion of depth by lowering the saturation or value of a corner
   in the back, you can improve the look of an icon with just a little
   work.

   For smooth shading, select the area you want to work with and then
   apply the airbrush tool. You'll only affect the selection, so you don't
   have to worry about overspray. To do hard highlights, use the one pixel
   pencil tool and, as before, a lowered opacity for the color.

Work Example

   In this small tutorial, Ximian artist Jakub Steiner will demonstrate
   most of the techniques described in the section called "A Few Tricks"
--
   but not completely white, color for the background layer. After you're
   done with the icon, you can drag colors from the palette to the
   background layer to make sure that the icon looks right on any
   background.

   Drag the black preview rectangle from the toolbox, or press Ctrl-+ to
   fill the silhouette. You should have a black square on a light
   background. Then, use the eraser tool to smooth the edges of the square
   so that the shape is better. This will also add a slight white shine to
   the corners of the image.

Making it Plastic