I'm also a fan of the anime style... it's often a good point between super realism and overly cartoony styles.
So: one thing with ArtRage (the full version) is the layers. They're IMMENSELY helpful, for things like stains or darker areas, where I'm not too sure just how dark to go.
So I'll paint a shadow area in black on another layer, and then use the opacity control to 'tone it down' to where it looks appropriate. Some of the blending modes are useful, too... play around, try them out.
Also, the Chalk and Crayon tools are fantastic for blending subtle tonal changes. Use them with very light pressure, and with the 'grain' of the paper turned off.
WARNING: ArtRage can cause serious loss of time, and excessive smiling! Use with care! Frequently!
“All creativity is an extended form of a joke.” – Alan Kay