If it's freezing then just don't do anything, except when you can really hose down the car and put it in a heated garage. Adding little bits of water will just activate the salt and/or freeze.
If it's not freezing then hose down the car with a LOT of water and also spray the underside. In general always hose down the car before you do anything. All the crud will make for a nice abrasive when you start rubbing down the car with cleaner, wax, whatever. After the hose down you can use your favorite wash, cleaner, etc., etc.
I never do much cleaning of my cars during the salty days. Just before winter I give them a last polish and a nice layer of good quality wax (also on the rims) and that's it until spring. The only exception is when I have to go to Boston. Driving around there is like driving through the sea, the amount of salt they put on the roads is beyond belief!
Stay away from car washes, especially the touch-less ones. They all are using really aggressive chemicals that strip all protection from your car.
Of course the best is to store your Boxster for the winter. Every springtime you have that exciting moment that you taking it out for the first time in months
Argh, I hate winter
