I'm yet to be convinced of the electronic rust prevention things, I have seen rust starting right next to one of those gadgets and that was on a near new car!
I'm a panelbeater so I get to see a bit of rust in my job and it mostly occurs where water and dust settle, ie in the bottom of panels, so just make sure to keep the drain holes clean. Liquid rust proofing(lanolin,wax fish oil etc.) puts a coating on the inner surfaces which works well. It also helps if the factory puts enough paint on it to start with too. Ford was having to do a lot of warranty work in the last five years or so because they just hadn't painted their cars properly!