From Donald Crawley, a Golf Magazine Top 100 teacher, director of The Boulders Golf Academy in Scottsdale, Ariz., and president of GolfSimplified:

If you pull your irons to the left, check your ball position. The ball may be too far forward in your stance, which will align your shoulders open and to the left of the target.

After aiming the club face, take your stance with the handle pointing in line with the center of your body. This will place the ball forward of center, but not up on the target foot. A bonus of this ball position is that your shoulders will be aligned square, not open. A combination of correct ball position and square shoulder alignment will fix that pull.

See Also:
Video: Correct ball position
More on the proper stance and correct ball position