Might be a daft suggestion but check that:
1. You don't have 'air recirculation' set on your aircon panel.
2. The pollen filter isn't blocked, which causes water to be held & easily mists up the screen.
3. Your interior screen isn't dirty. That tends to leave a film of dirt to which moisture can bind, rapidly increasing the tendency to mist up. I find a bit of Autoglym Car Glass polish (not Fast Glass) does the trick.
As Mike says, Rain-X external glass treatment is awesome.