http://cars.about.com/od/adviceforowners/a/ag_fightrust.htm
- When the temperature is below freezing, do not wash your car. The ice and snow will hold the salt in suspension and keep it from doing any major damage.
- As soon as the temp climbs above freezing, take the car to a car wash that has an underbody spray. Be sure the car wash uses fresh and not recycled water, otherwise you're just having salt from other cars sprayed onto yours. If the temperature stays up, wash the car weekly.