Most winter damage happens because a bike is put away wet or salty, then sits in a cold garage where condensation forms every night. A couple of hours of preparation stops nearly all of it.
1. Wash and dry it properly
Road salt keeps corroding metal all winter if you leave it on. Wash the bike thoroughly, rinse with plenty of fresh water and dry it. Then go for a short ride to drive water out of the hidden places, or use a blower. Pay attention to the swingarm pivot, the underside of the engine and the backs of the wheels.
2. Chain
Clean the chain with chain cleaner and lube it with Bike X Lube or ceramic chain oil while it's warm, so the lube soaks into the rollers. A dry chain sitting all winter rusts at the links.
3. Protect bare metal
Wipe or lightly spray Ballistol over exposed fasteners, footpegs, levers, the side-stand pivot, chrome trim and any bare alloy. Wipe off the excess so dust doesn't stick. A light spray into switchgear and connectors helps keep moisture out of the electrics.
Keep it off the brakes and tyres. Cover the discs and calipers before spraying, and if any oil gets on them, clean them with brake and parts cleaner before you ride.
4. Battery
Fit a smart trickle charger (an optimiser), or take the battery out and store it somewhere frost-free. After cleaning the terminals, a light film of oil on the outside of them slows corrosion.
5. Fuel
Fill the tank to the top to reduce condensation inside it, and consider a fuel stabiliser, especially with E10 petrol. Run the engine for a few minutes afterwards so treated fuel reaches the carbs or injectors.
6. Tyres and stand
Put the bike on a paddock or centre stand to take the weight off the tyres, or move it every few weeks. Inflate the tyres to normal pressure.
7. Cover it
Use a breathable indoor cover. A plastic sheet traps condensation against the paint and chrome, which is worse than no cover at all.
In spring
Check the chain and re-lube it, wipe down the bike, check tyre pressures and brakes, and make sure there's no oil anywhere near the discs before your first ride.