Don't allow the tin worm to set up house in your vehicle. Inspect every year with a bright flashlight, mirror(s) and patience. When you spot rust, disassemble as far as you can, remove the rust and clean thoroughly, and attack with POR15 or other rust-abating coatings. Seal leaks and holes with spray undercoating to keep water out (make sure you don't make a moisture trap). Maintain seals around doors, windows, trunks, etc. If rust develops in areas you couldn't see, fix it when you discover it! If you keep on top of it you can keep the vehicle going for longer than you'll need it.
I pulled the rear bumper on my 7-year-old truck this spring so I could attack the very bad rust that was eating it away. How many trucks have you seen with rusted-through bumpers? Mine won't be one of them. Front bumper is due as well.