Salt itself isn't a corrosive agent (rust is FeO so only compounds containing oxides, like water or acid, will directly corrode iron/steel)--it's a hygroscopic electrolyte, which makes it a powerful catalyst for the corrosion process. In other words, it actually attracts water (the actual corrosive agent) into contact with the iron/iron alloys in your car's chassis or body panels and then, as an electrolyte, aggressively facilitates the electron transfer that characterizes the iron corrosion process.
Fortunately, you won't have a corrosion problem unless the salt is allowed to accumulate and then remain at temperatures above the freezing point of water (ice is comparatively inert). Above freezing, the salt attracts liquid water vapor from the surrounding air which then has a chance to start working on exposed iron or iron alloys (obviously, galvanization slows or otherwise prevents this, since the zinc coating prevents water from coming into contact with the iron underneath). Just take care to spray exposed areas thoroughly with fresh water, particularly if the car is parked or stored in a heated environment or the outside weather is warming to a point above freezing. Salt is also highly soluble, so throw enough fresh water on the car and it will literally wash away. Water won't aggressively bond with the iron without the presence of an effective electrolyte like salt, so fresh water is OK.
Cosmoline isn't a bad idea, though it has a habit of trapping grime. As a hydrocarbon base, it (like any oil) is hydrophobic and will also form an additional chemical barrier between the car and any water that may be nearby. Rifles, machine parts, tools and other iron materials going into long term storage are (or were) often coated in cosmoline, which is effective for decades. You might also consider using a teflon spray on some of the underbody parts and panels. Also, practice good general care techniques: as always, keep your paint in good shape and use a good wax or polish; this will greatly help prevent the development of surface rust on the visible exterior.
Oh--apologies if there is any incorrect or misleading information above--my background is in physics, not chemistry, so I'm reaching all the way back to the general science requirement of my undergraduate education...regardless, with a couple years of hands-on experience in rust-proofing rifle bores, I trust my methods implicitly. Just use ample amounts of fresh water above all else.
