Gold plating is ideal -- and expensive. Tin plating is easy and more affordable. Normally, after cleaning all the flux from the solder-side of the PCB, I just shoot the back of the board with Krylon Clear Spray. It's a thin-enough coat that you can easily do board repair (use clear fingernail polish to recoat the repaired area) yet thick enough to stop oxidation. Unless the board is being used in a nasty environment, corrosion reduction just from exposure to air is more a cosmetic concern. Board with bare copper made in the 1960s still work fine today.