Copper left out in the elements, especially with added guano (birdy doo) will inevitably dull, getting darker and eventually turn greenish. I can't imagine trying to prevent this as it would likely involve a lot of cleaning and polishing. Then you would be tempted to coat it with something to protect the shine and you would never be happy. I would just try to resign yourself to the fact that it will be dull and get a patina. I usually use lead coated copper for flashing details etc. b/c of the salty air where I live here in New England. The lead protects the copper from excessive corrosion from salt and acid rain (and guano). Keeping the copper polished will actually wear it out sooner as the patina serves to protect it somewhat.