I think the most important thing is to keep it clean. If you drive through salty slush after a storm, get it washed, most importantly the underbody. I think oil spray helps too but I wouldn't bother going to a rustproofer. The stuff costs money so they use as little as they can (Rich from Luxury said he took apart a car that was supposedly Krowned and there wasn't a drop of anything in the panels, even though holes were drilled). What I want to do sometime is buy a tub of the stuff between a few people, find a lift and just do our own cars.