The Extension Bench
The extension position - a point halfway through the backswing where the club extends directly away from your target - is an extremely important movement in the swing, as it is here that the length of your swing arc (and thus your power potential) is established. Too many golfers fail to achieve a proper extension when they either cock their wrists too soon on the backswing or pull the club to the inside and around them rather than pointing it straight back.
Bruce Edwards gave me a good tip for working on the extension. It requires a prop, but one that you'll find on many tees - a bench. Take your stance on one side of the bench with your club extending across to the other side. Then practise making a straight-back takeaway of the club, as far back as you comfortably can. The bench will not allow you to pull the club to the inside, so you'll quickly get the feel of how to establish a full, powerful arc.