Exactly what Scott said but add inspect impeller and replace every 2 years or earlier as necessary if the fins become ridged, fill up the fuel tank and add stabilizer, pull batteries and put them on a trickle charger/maintainer, and run fogging oil through the engine (if you want).
In the spring be sure you change the water/fuel separator, grease the driveshaft and u-joints after inspecting them, and don't forget to grease your trailer bearings!
Here is the checklist I use: not all inclusive but it will get you started.
Winterization Proceedures:
- Fill gas tank and add fuel stabilizer
- Change engine oil & filter
- Change water/fuel separator
- Check trim pump level
- ATF fluid
- Check power steering fluid level
- Drain cooling system
- Inspect u-joint and exhaust bellows
- Replace lower gear unit oil
- Pull outdrive and service
- grease u-joint bearings
- grease u-joint splines
- inspect gimbal bearing
- Impeller ? Inspect / Replace every 2 years
- Pull prop and inspect
- grease shaft
- Lubricate gimbal and all zerks on transom housing
- Clean terminal ends on battery
Summerization Proceedures:
- Inspect trim pump fluid level
- Inspect engine oil level
- Check gear unit oil level
4. Inspect carburetor fuel filter