What I have done in the spring is using warm water, pouring a little water on the stained areas and then using a wet vac, sucking out the salty water. I do this 3 or 4 times, then leave the windows down and let the areas completely air dry. You want to get the salt out as after a few years it can start to rust or corode what is beneath the carpet.
Best idea is to start with either Weather Tech or Husky custom molded floor liners, as they keep the salty muck from gettting into the carper in the first place. You need to check them, because if muck does get below them, the solid plastic liners keep the wet areas from drying which can cause even greater rust/corrosian issues