Spray moisture-displacing fogger "stuff" in the plug holes and rotate the crank to expose the valve seats while you're doing it. While it's the off season, rotate the crank 90* every month to ensure the same seats & valves aren't exposed the entire time. Plug or tape shut any exposed holes (carb, exhaust) to keep the elements from getting in.

I have first-hand experience w/ what happens when you DON'T do this; it required pulling the heads to get the valves & seats touched up that had rusted during the down time... totally self-inflicted.