Go to Harbor Freight and pick up one of their rock tumblers. They make a single and double drum models. I can't speak to the double model, but I've used the single drum for years.
The real downside to these is tve drive belt, they're pure crap. You can either buy an O ring or what I've had forever is an O ring from a used Ford diesel oil filter. It works really well. Fill the drum with cases, cover with hot water, add dish soap and a touch of lemi-shine. Like new cases.
I've used one of those Harbor Freight vibratory tumbler and the same 20# bag of lizard litter from Petco for years now. It works like a champ.
Instead of a cylinder that rotates on rollers like Metryshooter described, the one I have has a bowl shaped like a bundt cake pan. Fill it up about halfway with lizard litter/crushed walnut shells, turn it on, then add enough cases to be cleaned. The one I have easily holds 500 9mm cases.
Then I use a 5 gal bucket and a $3 collander to separate and sift the brass.
