Light oil (which is a bit messy) wiped over them, a good quality Carnauba wax, clear lacquer, all of these will work.

Can't see where you live, but having nothing on them at all if you live where it's a nice and dry climate will mean that they will dull only a little, but if your somewhere with high humidity or it's always raining will mean that even indoors, they will dull up and tarnish relatively quickly.