RI (our home state) has the dubious distinction of being the "saltiest state" in the country. We use more road salt per lane mile annually(44.5 tons IIRC) than any other state in the country.
It shows on all the rotten, rusty, undercarriages around here, as well as our crumbling infrastructure.
My solution to the salt problem is multi-tiered, but has worked very well for me as I have a 2005 Jeep Wrangler that I bought new and still has a spotless frame.
First, washdown as soon and as often as possible. Water and car soap are fine, but make sure you are getting up in all the nooks and crannies, and especially inside the framerails. You will have to get right up under there and you will be soaked.
Next, are the semi-annual oil baths. I tend to do them in the spring after all the salt is gone and and again in the fall around halloween. After a good wash, I will use either WD-40, hydraulic fluid, motor oil, or a combination of these or other oils. Put them in a spray bottle, park over a tarp, and coat the chassis, suspension, body tub, hell everything but the exhaust with a light coating of oil and let it stay.
Learned this trick from an old farmer, and this has kept my vehicles rust free for the most part.
I did give the Supahcoach a light oil bath when she got here this spring, but I won't be driving her until all the salt is gone and before then she will get another WD-40 washdown. I buy it in the gallon jugs and use either a 1/2 gallon hand spray bottle, or a little hand pump sprayer.
Also, if you have a garage or heated parking space, NEVER park inside before washing off the salt, for some reason it seems to accelerate the rusting process.