This is what I use, every two years:

http://rustcheck.com/

Unfortunately, it isn't available where I live (southern NH) so I get it done in Canada while up visiting family. Lately, I've been thinking about buying in bulk and applying myself.

The stuff works. Places where rust would normally form on a 1989 Toyota truck are rust-free on mine - rockers, doors, back corners of cab, tailgate, bed. In 02 I cut away part of the bed with a sawzall near where the exhaust exits because a rotted exhaust blew on the sheet metal for too long and rotted a small section out. I did not paint or otherwise treat the cut metal. Just this year a bit of rust is forming around the cut where the rustcheck oozed out because I went 3 winters instead of two without getting the body treated. I wish there was somewhere around here I could get this done.

Some people recommend a product called Fluid Film for rust prevention. I haven't tried that myself. I think I read about it on plowsite.com.