Walnut from the pet stores or Harbor Freight ( they sell it for sand blasting ) and Nu Finish car polish. For really nasty dirty brass I'll use the chrome polish that you would nomally use on a cloth wheel, just run the brick of it on a cheese grater to get a powder form of it and throw it in with the walnut media, works great.
I once needed to clean the tar off of surplus 30 cal projectiles so I used corncob and acetone, do this outside away from everything. Added the corncob and the projectiles, turned on the tumbler, added the acetone ( leave the lid off ), came back in 15 minutes after the acetone had evaporated and the projectiles were perfectly clean. This also made the inside of the tumbler clean as new.