I have used screen covers in the past with patchy success, some plastic ones leave a film on the screen that smears when wipers are used. Covers that are not impermeable to water can get wet and stick to the screen. A piece of polythene sheet is as good as anything ( had a new mattress lately, they come wrapped in acres of polythene) - make it long enough to tuck into the front doors either side of screen to keep it in place. Of all the de-icers I have used I found AutoGlym works the best, just a small amount required ( it is quite a fine misty spray from the bottle ) and it does not smear the screen like a lot of others do ( if you have an old de-icer spray bottle you can fill it up with regular winter screenwash, it works out cheaper). Don't like full car covers as the car can 'sweat' under the cover and promote corrosion. The real answer is a carport, which lets the air blow around the car but keeps frost and snow off it.
Always keep your windscreen washer bottle topped up with decent winter screenwash ( prestone or Comma -25 stuff ) don't use the -5 stuff as it can freeze in the nozzles and if you get the lower temp stuff it acts as a good de-icer as well, many mornings when I was off to work at 4-15 I would clear the screen only to have it freeze up again as I was driving, the normal -5 stuff just used to turn to slush on the screen and freeze up again.
Using rainex on side windows makes the ice and frost easy to get off ( like teflon coating the glass) but using it on any screen with a wiper means it will cause smears and the teflon effect wears off in a short time. Don't forget to clean off all your lights and side and rear windows and the roof as well ( if there is snow on it, it is an offence to drive with snow on the roof).
