Run the A/C and make sure it's not on recirculate, but on outside air. Not sure if you have the car in a parkade or not, but I do and the air there is moist and warm from all the runoff, which means the air in the car is too. Going outside it condenses against the now cold windshield and ices over. YMMV.
What kind of car is it? And stop making out in it!
