Media and Polish
I run a Lyman 1200 tumbler and I use 2 lbs of walnut that I get from a pet supply (cheap, 7lbs for $9.00) I put my dirty brass in the tumbler and let it run for about 2 hours. I then add 4-6 dabs (little finger-nail size) of Mothers Aluminum and Metal Polish. (comes in a red and white tub, $5.00) Another two hours of tumbling and the brass looks better than new. The "dabs" of Mothers quickly disapates into the media. The next batch or two of brass just needs 2-3 hours of tumbling and seldom needs anymore Mothers.
This formula has worked great for me for many years.