I have had a fair few problems with my Altea steaming up and the Leon before it as it happens. I've found that although the air con clears the screen, it's as if the water condenses elsewhere in the air con system and is held there until you next run the blowers (without air con).
So I try and run the air con as little as possible, and if I have I will try and dry the system out by running the blowers at full heat and full fan for a couple of minutes when the car's warm.
This minimises the
problem for me.
Jon