Warm up your engine, then after shutting it down, drain the old oil from the crankcase and replace with new. Letting the engine sit over the winter with old oil will cause acids and other corrosives in the stuff to damage interior components.
Drain the raw-water cooling system entirely—you don’t want water in it freezing or causing corrosion. Add a 50-50 mix of propylene glycol antifreeze to the system (as much as possible) via a funnel at some convenient spot, usually in the vicinity of the raw-water pump. You’ll have to pull off a hose temporarily, of course. Let the antifreeze stand for a half hour or so and then drain. The antifreeze coating prevents corrosion.
Top off your fuel tanks and add a biocide.
Remove your raw-water pump impeller so it doesn’t deform or suffer damage from antifreeze residue.
Make sure your zincs are current.
This article originally appeared in the December 2012 issue of Power & Motoryacht magazine.