"Casting" is releasing the club from the top of the swing. Your wrists unhinge early, and your club resembles a fishing rod "casting" a lure to a pond behind you. Casting results in loss of power and accuracy.
To stop casting, think of your body as a hurricane. Your arms are like the outward arms and your body the middle of the storm. As the head stays still (the eye of the hurricane), the body uncoils. The arms follow behind but at extremely high velocities. Your upper body effectively pulls your arms and thus, the club, into the ball.
This tip was inspired by the January 1 entry in Bill Kroen's "Golf Tip a Day" calendar. He's also written a book, which you can find at Amazon.com, Barnes & Noble, and other book retailers.