As I promised to some of the people here, I'm releasing the first tutorial here.
This is the tutorial for how to do soft cell shading on Photoshop, based on my experience.
The version of Photoshop used in the tutorial is 6. If you use later versions, names of tools/modes and/or allocation of shortcut keys might be different.
I'm working on Windows, so if you use Mac please change these notations as: Ctrl to Command, Alt to Option.
I just hope it helps you.