If possible, store your boat ashore for the winter. The bulkhead, keel and motor are the critical areas needing support. Cradles work best, but don't store your boat on a cradle that was designed for a different model.
If you store your boat in the water, close all through-hull fittings, gate valves and seacocks. Do not close cockpit drains. Plug exhaust ports. Check your boat occasionally to make sure lines are secure, bumpers are in place, and the bilge is dry.
Cover the boat using boat covers. Use a frame under the cover to prevent water from pooling and tearing your cover or damaging the boat. Canvas is best because it breathes, but plastic works too. Make sure you leave vents in it to allow condensation to escape. Allow for drainage if you're storing outside.
Add non-toxic anti-freeze to water tanks, toilets and septic holding tanks. Never use engine anti-freeze in a freshwater system.
Remove electronic equipment, important documents and other valuables that could tempt thieves.