Check for water inside the car, you don't want any leaks.
As for HVAC, set to outside (fresh air) and keep it on speed 2. If there's no leaks then it should not mist up at all. Check the spare wheel well for water. Some have found a swimming pool there!
If you have airconditioning then outside air and ac on will do the trick and use the heat to "reheat" the air. You may get away with speed 1 here with ac on.
Here are some temperature observations:
10+c can require air conditioning to demist in rain.
5-10c can do either, aircon can help you drop the fan speed.
Below 5c air conditioning is less effective, if you have it you can still run it but to save a bit of fuel, put it off and use the fan at a higher speed.
ALWAYS use fresh air mode, the only times it is supposed to be used are these:
Air pollution (outside stinks)
Fast airconditoning (re-conditioning cool air enhances ac performance).
And in general have the fan on all the time, it should be silent enough on speed 1. There is a large difference between off and 1. It really slows down the misting.