The main thing to do is avoid any moisture ingress to the car, because that is what is freezing on the inside of the screen. When I lived through very cold winters in the past I used to open the driver's window so the moisture on my breath did not hit the ice cold screen and freeze. Yes, an auxiliary fan heater will help but will drain an awful lot of current that the engine might not be able to put back and which might leave you with a flat battery. A/c should dry any moisture in the air circulating inside the car so should be run on recirculate. Also make sure the pollen filter is not clogged with dirt or moisture.