Be The (Middle) ball - Here's a drill that will help you get better at making contact in the center of the

A very important variable to better ballstriking is "centeredness of contact." It doesn't matter how dialed in your swing becomes, if you're missing the center of the clubface at impact, you're going to have trouble controlling your distances and hitting the ball in your intended direction. Hitting it on the "sweet spot" not only feels pure, it will also help you to maximize your distance.

Here's a drill that will help you get better at making contact in the center of the clubface. Simply set up with three golf balls, and place each ball a few inches apart, with the middle ball being the ball you intend to hit. Make sure the three balls form a line perpendicular to your target line. Leave just enough room for the clubhead to pass through without hitting the two outside balls.

If this is too easy for you, here's a variation of the drill. From the original drill, move the ball closest to you forward approximately 2 inches and the ball farthest from you away approximately 2 inches. Leave just enough room for the clubhead to pass through without hitting the two outside balls. This aligns the three golf balls diagonally, which now forces you to not only make centered contact with the middle ball, but it discourages you from coming over the top and hitting the ball from too outside-in the target line and encourages you to swing the club with an inside path on the downswing. Either variation drill will help you hit the ball more in the center of the face.
 
Reference Resource: http://www.golftipsmag.com/instruction/iron-play/quick-tips/be-the-middle-ball-best-of-2014.html