You could buy the large popcorn that came in a paper bag. They allowed one free refill per bag, which they "marked" by putting a tear at the top of the bag length-wise.

Anyway, while sitting there in a theater, waiting for a movie to start, I see this dude open up the theater door, walk over to the garbage can, look around it for a sec, and then pull an unmarked popcorn bag out of the garbage... presumably to go get his one free refill.