I think Miller88 is doing about the best you can do.
Clean and wax the door jambs, under the door panels, spray into the hinges with some sort of lubricant. All your locks, trunk lid, hinges, shocks, literally under the hood. Then when you put your winter tires/rims on....maybe treat your suspension and steering, brake lines, hubs, etc, wheel wells, inner side of wheels.
Then after the salt is gone in spring. Use, I think cold water to flush alot of underneath the underbody.
This in addition to putting in an appropriate winter oil, checking (if necessary) the antifreeze ratio and put a piece of cardboard under your windshield wipers at night.
Good luck you Northerners. I hear dedicated snow/ice tires are well worth the price.