Covering windshields and car windows with carpet squares or other pieces of cloth keeps water from freezing to the glass. Covering side mirrors with plastic grocery bags or Ziploc storage bags keeps ice from developing on those, too.
The best way to keep water from freezing on car windows is to pull the car into the garage overnight, or at least to keep it under a carport. If that isn't possible on a night with frozen precipitation, covering the glass keeps water from adhering to the surface. Carpet squares and pieces of cloth are easy to take off before getting into the car.
It's more difficult to keep a piece of cloth in place on a side mirror because of the vertical surface of the glass. The small size of the mirror makes wrapping and tying a plastic grocery bag, or small trash bag, around the mirror more convenient to keep the water off the glass.
Some people recommend spraying a mixture of three parts water and one part vinegar onto glass to break up ice or keeping it from forming. However, vinegar is acidic, and while this might be an effective one-time solution, the acid corrodes paint and windows over time.