When I was in college, I dated a girl whose job it was to go to grocery stores and place the prize winning bottles. They had a well thought out system to ensure that the prizes were won across the board by everyone, but we just followed her and bought the drinks she messed with(she never did know that we did this, BTW). We lived for an entire summer off of the grocery promotion under those bottle caps, and I became a Pepsi drinker to boot!