Save your dirtiest cleaning patches in a jar with some ammonia. Just enough to keep moist/wet.
Clean the brass really good to remove oils, waxes, and any polish you may have in it.
rub the brass down with them saved dirty patches and the brass will tarnish pretty quick. To keep it that way, whenever you clean, wipe with the new dirty patches (or not). I've had brass come out brown, blue, black and sometimes a blood red (?) depending on the brass and amount (frquency) used.
And it's free! :thumbsup: