Ball position is everything in iron play. If you aren't careful about it, you can create some major problems in your game just by getting an inch or two off. One of my tendencies is to let the ball drift too far forward in my stance, and it's something I've been working on with David Leadbetter.
For a 7-iron, you never want the ball to be closer to your left heel than just slightly ahead of the mid-point of your stance. That's especially true if you're a tall player, like me. If you play the ball too far forward, you have to slide your hips toward the target to go fetch it at impact. You'll tend to make a big divot, and the shot will balloon into the air.
With shorter clubs, your ball position should be just back of middle, to really promote hitting the ball first on a downward strike.