Many amateurs try to help the ball into the air on iron shots. They think they have to get under it, so they pull up out of their posture and catch the ball on the upswing. This often causes them to make contact on the bottom edge of the clubface. Whoever coined the term "thin to win" never saw a 7-iron scream 40 yards over the green.
If thin shots are a problem for you, make two adjustments in your setup and one key move in your swing. Center the ball between your feet, and put a little more weight on your front foot. As you swing, make sure you hit down on the ball with your hands leading the clubhead and your weight entirely on your front side. This will help you maintain your posture—no more lifting—and you'll compress the ball off the turf.
Tension is a killer in the swing. Just thinking Relax doesn't cut it.
My advice: Without a club, get in your setup and pretend you're taking your grip. Feel how relaxed you are? Now re-create that feeling when you put a club in your hands. It'll give you a clear sense of keeping out tension.