Rust is not your car's friend. It will get in and start eating your car's body and frame away. It can get under the paint and do damage without you even seeing it. If you live in a place where you get a lot of snow and ice, you probably have some kind of salt on your streets, which will affect the bottom of your car and give rust a good place to chew up your car. Because it's under your car, you won't even know that there is a problem. So what can you do?
Wash Your Car
It may seem weird that washing your car can help get rid of rust, since water plus metal tends to equal rust, but there are reasons as to why you should keep your car washed. One of the reasons is that washing your car keeps things like salt and other substances off of your car. Keeping that stuff off will keep that stuff from getting a good foothold in your car. In the winter, you want to wash your car when it's above freezing, if at all possible. If you have to wash it when temperatures are under freezing, make sure that the car wash that you use has a dryer at the end of the cycle so that you can get rid of the water. When you wash your car, make sure that you are getting an under-body wash as well, to make sure that everything is clean.
Another thing that you can do is to get a rust coating put on your car. The car wash you use may have the ability to spray some kind of coating under your car to help inhibit rust growth. If not, you are going to have to go to an auto body shop and have them put a rust sealant on your car. If they do this, they will clean your car, make sure that any rust damage is already taken care of, and then spray the coating all over your car and the under-body of your car. The reason that they are going to wash the car and repair any rust damage is that you don't want to have any rust that can work under the coating.
You want your car to work as well as possible, and keeping rust off your car is one way to do that.