To prevent car doors from freezing shut, lubricate the locks on the vehicle. One lubrication option is to take a cooking spray or WD-40 and spray this onto the locks of the vehicle to prevent freezing. Another option is to apply petroleum jelly to the vehicle's key and then insert it into the lock to thoroughly lubricate it.
If the locks of a vehicle are already frozen, a de-icing spray may help to unfreeze the lock when sprayed directly into it. Applying petroleum jelly to the vehicle key and inserting it into the lock may also work to unfreeze it so that the vehicle door unlocks.