There are various brass polishes you can add to the corn cob media, Cabela's, Midway USA and other sites have them or you can just use a liquid car polish. I have used Nu Finish auto polish and the Cabela's metallic cartridge polish and see no difference between them. Add 4 capfulls to the corn cob media and run the tumbler for about 15 to 20 minutes to get the polish all mixed in good and there are no lumps left. Then add the brass cases and run the tumbler until you get the cases all clean and shiny.
I have not used corn cob media, instead I use lizard bedding from Petsmart, it is made of crushed english walnut shells.