Make sure your heater controls are up on the windscreen setting otherwise no air blows onto the door windows. As Andrew said if you have aircon then use it as it dries the air out and keeps the system working properly. If you don't use it during the winter the levels will drop and it won't work as well in the summer when you want it
If it's especially damp and misty crank up the heater full and open the windows both sides a bit - the fresh air will get in and the old moist air will be gone clearing them at the same time.
I get this all the time having to transport my 3 wet dogs around for their daily walkies so used to having to clear the screens in all weathers.
