The best way to prevent the build up of condensation in a tent is to leave your rain fly open at night to promote as much ventilation as possible. If water vapor can escape out a vent, window or door, you can significantly reduce or eliminate the amount of tent condensation that occurs. If it is raining you have an additional problem. The best solution for rainy conditions is to be sure that your rain fly is as far away from the tent as is practical. You can stake out your fly seperately to give yourself some additonal clearance. You want to create as much airflow between the tent layers as you possibly can without compromising the functionality of the rain fly.