Most people perform chin-ups without first getting into a stable shoulder position. This can reduce the efficiency of the exercise. You need to create a stable base, which requires a conscious muscle contraction.
Take an overhand or parallel grip on a chin-up bar.
From there, lean back, raise your chest using the thoracic spine as a fulcrum, and pull the shoulder blades down and back.
It helps to try to "break" the bar apart. This exercise also works tremendously well to correct trouble with arching during the bench press.
If a trainer were to come to us and say he had a method of ab training that would reduce the waist size of anyone in three weeks, we'd ask him what he was selling and then kick him out the door. Luckily, Don Alessi was a fast talker and before we could introduce him to the sidewalk, he'd convinced us that he had some pretty nifty ideas.