Auto detailing brushes work well for cleaning cassettes. You can get a pack of three (one angled plasticy bristle, two with metal bristles) for a buck and a half if they're on sale at target. Just take the wheel off, pour a little degreaser into a cup, then fill another cup with clean water. For degreasers take your pick, my latest favorite is Roundy's Green (the store-brand Simple Green at my grocery store). For the chain: if you want to make it a big hassle then you can take it off and soak it in degreaser but it's easier and works just as well to put some degreaser on a rag and run the chain through it several times making sure to scrub all of the gunk off out of the outside.