Stand facing a wall, about 5 feet back. Do a squat. Move a few inches forward. Do another squat. Repeat until you're too close to the wall to complete a full rep. When you fail it will be because of a lack of flexibility in your squatting chain, which causes either your knees or torso to go too far forward. This is a great warmup by the way. If you get to the point where you're doing this with you're feet against the wall, try it with a broomstick overhead. Facing-the-wall squats and overhead squats are a killer combination when it comes to improving squat form and flexibility; you can't do either well until you're flexible and have learned to rely on hip extension in your squats.
Previously Masquerading as Ross Greenberg