Use your air conditioner to dehumidify the air
Turn on the defroster
Set the temperature to warm or cool, not recirculate
Crack a window slightly
Clean the inside of the windshield
Replace a dirty cabin air filter
Remove wet items from the car
Dry floor mats and carpets
Check for leaks that let moisture into the car
Use an anti-fog treatment on the glass
Keep the windshield vents clear
Avoid breathing directly onto the windshield