Sure, Flitz will clean it up nice, but it never stays that way. Anyway i
prefer this cheap stuff called "bar-keepers friend" or something like that,
i think i get it at home depot or menards. Its a powder and is extremely
effective at ripping the oxide right off the copper, and as noted, its
really cheap.
I found that putting a lexan plate over my primaries tends to increase the
"lifetime" before it looks like crap, but i suspect thats mostly due to
handling (you dont get your oils all over it). Even still, give it enough
time and it will start turning darker.