First, don't drive it on the salted roads in the winter. And keep it in a garage, or failing that, under cover.
If you must drive it in the winter, you can apply one of the undercarriage treatments like Fluid Film (DIY), or Waxoyl. The more you wash it, the better. Treat any body cavities like inside the doors with cavity wax - available from 3M. I think you'll have to be resigned to it rusting after a few years, if you drive it regularly on the salted roads.
Maybe you'll want to buy a beater to drive in the winter?
1) '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps. 2) '82 J20 360/T18/NP208/3.73, KOs, 7600 GVWR 3) '75 CJ-6, 304/T15, PS, BFG KM2s, soft top 4) '95 VW Golf Sport 2000cc 2D