[BrunoThePug]

There was a drill in this months golf digest for this exact problem.

Put a tee in the ground and address it like you would the ball. Then put another tee in the ground about 1 club head further away from your body from the first tee. It should look something like this... (ignore the periods)

.....* <-- This tee is about 1 inch behind the original tee
......
...... <-- This distance is 1 club head
......
...*.. <-- Original tee

Address the original tee that's closest to you like it were a ball and you were going to hit it. But when you swing try to hit the tee second, further away, tee.

The point of the drill was to focus on arm extension through the down swing and at impact.