Its tough. Clean the chasis now by sanding any surface rust areas and paint the entire chasis with a couple coats of flat black spray paint. Por15 is a more expensive option that may yeild better results but a good heavy spray painting works wonders.
Remove the chrome trim on the edge of all 4 fenders and store them away - its hard to get the salt from under them and its a trouble spot.
Ensure the plastic inner fender wells in the front wheel wells
havnt warped and do not have gaps between them and the fender itself, it will cause the fenders to rust very quickly.
The rear hatch/door rusts very quickly fromm the inside out from salt - ensure when washing that you get water down the inside through the door seal a bit (with a wand) as its a real trouble spot.
The rear fenders are a no brainer trouble spot, pay special attention.
Every single time your truck gets salt on it, you must wash it. It gets expensive and its really anoying, but you will build body rust within one winter on these old trucks if you do not wash liberally.