I didn't see it mentioned but you can use a product like 'RIG' grease for guns you don't plan to use anytime soon. With guns I have no intention of shooting for years going forward I apply a light coating of RIG grease in the bore & on the outside before storage. I always clean any gun I put back in service, so I don't worry about a little grease in the barrel, no reason to overdue it though.
Besides that, a lightly oiled cloth & a damp patch goes a long way towards preservation.
