I use Nevr-Dull, a wadding treated with a Brasso-like chemical...Spin the cartridge in my hand, or if I'm lazy, spin it in a drill chuck...And the wadding removes old oxidation...Works great...
I usually do this for cartridges I need to display to acquaintances, but if you're planning on displaying yours with your pistol, why shine them up??? Just wipe them off and let the patina do the talking...