I don't have a canvas and pole, carport, lean-to, shed, or barn to store it in. I use my tractor all winter for snow removal and before we get too much snow, moving logs out of the woods; so it doesn't sit. Like several others have said, putting a tarp over the whole thing holds in the moisture and promotes even more rusting and condensation. So here's what I do.
1. Keep the leaves off the tractor. We have a lot of oak and those leaves are very acidic. Don't let them set on the tractor for any length of time. Clean them off at least once a week, daily is better.
2. Rust spots: for body and structural parts, scrape the rust off or lightly sand the spots, then hit them all with rust compound. It basically chemically bounds with the iron oxide and forms a plastic-like coating that can be left as is, or painted over. For your hydraulic pistons, don't let them get rusty in the first place. Keep them well greased. I know with my Bobcat's FEL, when the bucket is at full rest position, all the rams are completely retracted in their cylinders and not exposed to the elements at all. You can touch up a slightly rusty piston with ultrafine sandpaper, or buffing compound, but you have to thoroughly clean them afterwards before regreasing or using, otherwise the grit stays on it and it just keeps wearing. Don't sand too heavily or you can damage the piston; and getting them repaired is EXPENSIVE, at least to me.
3. If you have a cab, you're golden. If not, you can cover the seat and control area loosely with a tarp just before it snows if you want to make things easier to clean off. Otherwise, keep a soft broom handy for cleaning it off. I've even seen a couple guys use a leaf blower to clear powder off their tractors.
4. Park it on a place that drains well. You don't want your tires or attachments to get stuck in or to the ice and ground. Also, make sure your tires are at proper inflation. If they're a little low, that's more surface area to get stuck to the ground with, and you lose structural strength of the tire; which means pulling loose from the ice will cause more damage than if properly inflated.
5. Make sure your coolant antifreeze is good, and all your lubricants are full. I recommend keeping your fuel tank full to minimize condensation area in the tank. Algae growth in fuel is pretty much a non-issue in the winter, assuming you don't use it that much. In southern NH I'm not worried about the fuel gelling up in the cold. Up in the far north, or high country, that might be an issue and you might need an additive to keep it flowing properly.
Next year I should have enough logs cut to put up a pole barn shed against the side of the garage to store my tractor in.