Before putting the cardboard on, smear a jar of petroleum jelly on the windshield so the cardboard will not stick.

*Don't actually do this.*

EDIT: [For those asking "why?", someone explained it lower in the comments](http://www.reddit.com/r/AdviceAnimals/comments/2rqnml/as_a_northerner_living_in_the_south_this_morning/cnigp4b)