Best way is to not drive it in winter but,,,,,,,,,,,,most important is to keep it clean, especially the underside and all the nooks and crannies. Easier said than done, I know. At the very least, an automatic car wash that has an underbody flush. Where accessible, you can spray WD-40 into the seams such as the door bottoms, in the rear wheelwells (go in through the trunk), etc. Before doing that though, you may want to look into a product to rust proof the seams first. Eastwood makes a product that is fairly thin and will seep into the seams and pinch welds and seal the metal.
It's kind of stinky for a while after spraying but the odor goes away eventually. It's not a bad smell, just a smell.
Heavy-Duty Anti-Rust in Amber - Rust Proofing Products - Eastwood
There's also this, for inside the frame rails, rockers, etc.
Eastwood Internal Frame Anti-Rust Coating, 14 oz. Spray