I've had a bit of steam up recently, normally its just if you have had anything wet in the car in recent weeks or got in with a wet coat etc...
The trouble is that the car is kind of a closed environment so i find that if you have had steam up for a while sometimes its good to crank the heat up and have a window/sunroof open a bit when driving once in a while, this allows the hot (moist) air to escape and should reduce the issue in future days.
The other thing is to keep the inside of the screen really clean as this helps, as do the variius anti-mist products you can buy,
cheers,
R