The keys to hitting a long iron well are tempo and a low, sweeping move through the ball. Position the ball slightly farther forward than you would if you were hitting a shorter iron—generally keep it no more than two ball widths behind the left heel—and make a conscious effort to take the club away slowly. Never snatch.
Before I start my backswing, I like to visualize the area between eight o’clock and four o’clock and "see" my arms swinging fully extended throughout that range. By contrast, when using a short iron your wrists may already start to cock before the clubhead reaches the eight o’clock position in the backswing.