Theresa in Grand Valley Ontario is not looking forward to keeping a nice brass backsplash in her kitchen nice and brass looking.
The key is to clean it well, making sure there is no wax on it, and then spray it with a varnish or any other clear finish. Simply some transparent finish from a spray can. Clear Lacquer is usually used for antique brass placques. That will keep the oxygen and other air borne pollutants away from the brass for a long time.
If it needs redoing in a few years, usually just around the edges where the oxygen snuck in under your finish, simply use paint striper to take the clear finish off, clean the brass and then finish it again.