I would change the pollen filter and think about checking for leaks if its that bad. Air-con will clear the screen quickly but I often find it causes its own problems becuase as soon as you switch it off again the condensation comes back worse than before - unless you ramp the heat up and open the window to let the damp air dissipate.
When you press the de-mister button above the rear windscreen heater it automatically puts the aircon on. Or it does in mine anyway. I'm not sure it would be suitable on something as large as a windscreen but shaving foam stops it misting up. Just wipe on and keep rubbing till its clear but not so much that you remove it. My mate uses it on the inside of his bike helmet and I use on the inside of my goggles. Works a treat.
Can anyone confirm that when they press the screen clear button that they can't have the auto light on at the same time? Does shaving gel work... don't use foam