Since few After Effects users can do classical cel animation and a lot of them do not have a chance to use a 3D package's cartoon shading, here's a technique from Mylenium that will allow you to create such effects with ease inside After Effects. While it's quick and easy to setup and animate, it consumes a lot of time once it's time to render for final output. It also requires some major steps in Illustrator or Photoshop.
Adobe After Effects
Click on the link above to view Mylenium's tutorial here at Creativecow.net.