Gentlemen
It seems to me we have two questions being answered here - how to get brass shiny and how to keep it shiny. Frankly, I didn't care that my brass was dull and dingy until I got some shiny, then I wanted it all shiny! Shooting BP in them only makes the problem worse.
So far I've been successful with dry walnut media in the Lyman vibrator after cleaning them in ceramic media and dish soap in the vibrator. I'm new to this polishing bit, so I'll see how long the walnut media keeps working, but it's cheap. I keep my brass in cartridge boxes in ammo cans for the most part, so they stay the way I loaded them.
Wayne