I periodically use steel wool to clean and brighten my copper chiller. I bought it back in 1990, and it set unused for about 10 years. When I got it back out to start brewing again, it had some discoloration. Steel wool works well to clean up copper.
I agree with the other posts here: try vinegar (I am going to now), and there is no need to sanitize it before use, just drop in in the boil about 15 minutes before end of boil.