Recently, Ive picked up a few things from following other illustrators. One of the newer things Ive learned is the how to use the polygon lasso tool to make clean and subtle shading. The polygon lasso tool easily lends a more finished clean look as well as eliminates the need for line work, which can often flatten your illustration. Here is an animated gif I did of a friend of mine. I used the polygon lasso tool on a layer mask and went over the sections lightly with the brush tool.