Mark Kennedy on the Temple of the Seven Golden Camels blog quotes Ollie Johnston... When a drawing isn't working it's always tempting to clean it up in an attempt to "fix it" when, really, you know that the drawing is flawed and you should just start over.
After marking some student work this year where the direction to "work ruff" has been applied with gusto, this discussion about clarity in ruff drawing is very important to highlight.
There is also some great stuff from Glen Keane on the Drawn blog - another post about drawing for animation. Glen Keane on drawing in 3 dimensions.