how to avoid turnish (coper rusting)?

Answer 1

Option #1: Leave the object in a vacuum, like outer space.

Option #2: (equally practical as #1, and will only slow down the rusting, not stop it) get the industrialized nations of the world to stop polluting the atmosphere full of nitrogen and sulfur oxides which form acid rain that corrodes copper.

There is no option #3 if you're talking about things outdoors. Indoors you will still have the "copper rust" forming--don't leave acidic things in the copper, and scrub it down every once in a while, because it will still turnish just because of the O2 in the atmosphere.