I would use a grease remover (any suggested elswhere in this thread -- I use mineral spirits since I have that around anyway) and a toothbrush on the cassette. Don't forget your rear der pulleys, which should be removed for cleaning.
For the chain, maybe dampen a rag with WD-40 and wipe just the plates (sides) of the chain. I never clean a chain more than that (or a hose-drip after a rainy ride).