The suns UV rays are your paints worst enemy.  Park in the shade whenever possible and always keep a good coat of wax on your car.  The wax should have UV protection just like the sunscreen you use at the beach. Preventive Maintenance:

Wash the underside of your car during and after the winter season. This will wash off the salt used to melt the ice on the roads.  Folks in the southern and most west coast areas don't have this to worry about.
 
Make sure all drain holes around the trunk and hood are clear.  These holes allow water to exit the ledges around the trunk and hood and prevent rust.  Also check your weather stripping around all openings for rips and tears.  Water will sneak in where you least expect it and eventually rust away your car.

Waxing your car will keep your paint protected from the elements but what about the other side of the body?  The inside is often neglected on older cars.  Rust proofing is an option but if the rust has already started just covering it will not stop it.  The existing rust must be removed or chemically converted before rust coating is applied.