You may have heard this tip with regard to preventing a bathroom mirror from fogging up after a hot shower, but the same concept applies to a car windshield in winter. Smear a dollop of shaving cream along the inside of the windshield. Spread it out in an even layer; then use a clean cloth to buff it away. (You may need to use a few clean cloths and some elbow grease to remove all of it.)
The shaving cream creates a protective coating that won’t fog up, keeping your driving vision clear during really cold weather, and when the heat is blasting inside the car.
As mentioned above, this tip also works for mirrors, so you can follow the same steps on your vehicle’s side-view and rear-view mirrors.