The key to striking a pure iron shot is to hit down on the ball, resulting in a divot in front of the ball. Most players aim at the back of the ball, resulting in 2 things: not hitting the ground in the correct place (if at all) and not hitting the ball first. It is imperative to hit the ball then the ground, to accomplish this, the club must have taken a descending path. There are many good drills for this, I have included my favorite below, note that this applies to full swings and short game shots hit with an iron or wedge.
Place two golf tees about 9 inches apart pointing directly at each other. Place your golf ball in the middle with the tees pointing at the middle of the ball.
When you make your swing, attempt to make your divot appear in front of the tees, it will feel to most players who have never done this that you are going to hit it low, however, you will notice that the ball actually comes out higher, and will be a more efficient motion. When you do this, make sure the divot is straight at your target, not pointing left, this only works with a straight or inside out path, if the divot points left, you are coming over the top (across the line).
The writer is the head golf professional at Bass Rocks Golf Club in Gloucester.