preserving new vehicle?

What insight do you have to keep the sheet metal on a new vehicle from getting eaten by the rust moths? It "seems" that where I sprayed a previous truck with graphite spray because the locks would freeze at 31 degrees, that door and tail gate were fine. The passenger door, which I didn't use often...it's has issues. Not flapping in the breeze but you don't want to run your hand along the edge. The outer skin on the bed, no problem. Anytime it was above freezing for any period of time, I'd get the hose out and blast the snow and salt clumps away. The bed itself is unsafe to step in without putting a liner in of some sort. ( I cut one down from a different truck just so I could haul stuff w/o it falling thru ). Any sprays? Aftermarket undercoatings? Old engine oil? What is your favorite method? Thanks!