the opacity masks? I don't use cell shading.... the only example i have up is the face of [link] i lay down flat colors, them shade/highlight with layers set to clipping masks that are in muliply/screen/normal/etc. and go to work with my tablet (or the school's cintiq's - i am so spoiled). i have a much better example of that, but it's unfinished so it's not up yet. it uses the same technic, but looks way better than that example. i guess i don't think this is the best aplication of clipping masks, but it's still a useful tut because people should know about clipping masks, because they're very handy.
In the Layers Pallet, you need to right click the layer and one of the options will be Create Clipping Mask. Other options would be selecting the layer and either go to the top of the screen and click Layer > Create Clipping Mask or pressing Alt+Ctrl+G. Hope that helps. ^_^