Aim your chin at the ball.
When I was 15, my dad and I went to the 1973 Open Championship at Troon. I followed the winner, Tom Weiskopf, all week. What impressed me was the incredible sound he made at impact with the long irons, a crack nobody else made. To make precise contact like Weiskopf did, you cannot allow your head to drift around during the swing. He appeared to aim his chin at the ball at address and then kept it steady throughout the swing, letting it swivel slightly but never allowing it to dip. I've used that thought throughout my career.
Think direction over distance.
When I won the 1990 Masters, I don't recall missing a single green to the left or right. I missed only short or long -- a secret to playing Augusta National not all players are aware of. It all goes back to my days as a teenager at Welwyn Garden City Golf Club, in England. It had a short, narrow practice area, no more than a 7-iron shot in length to a small target green, and naturally I developed an iron game more oriented toward direction than distance. Controlling distance gets most of the emphasis these days, and I appreciate that, but the mark of a pure swing is whether you hit the ball dead on line. If you can learn to do that, your distance control will follow eventually.
Friction between the ball and clubface is what makes the ball spin, and compressing the ball is what makes it go forward. To make your short and middle iron shots sing, you need a blend of these two factors. It's a delicate balance, but one way to achieve it is to imagine the clubface being made of sandpaper, and keeping the ball squashed against that abrasive surface for as long as possible through impact. Scientifically it probably isn't possible, but still you should try. Two ways to do it, which I learned by watching Lee Trevino: Keep the clubhead moving down the target line for as long as possible, and make sure your hands are ahead of the clubhead through impact.
Mimic the best players.
After I came home from that 1973 Open, I'd play imaginary three-balls. In the "group" were Weiskopf, Johnny Miller and me. I'd copy Miller's fading ball flight and even the way he walked into the ball at address. I tried to copy the divots Weiskopf took with the long irons, which were exactly the length and width of my forefinger. Imitate the best iron players. What will emerge is a swing that is part yours, part theirs -- a winning combination.
Finesse the short irons.
I played with the great Billy Casper at a tournament in Kenya in the late 1970s. His play with the short irons was as good as any I've seen since. He didn't impart tremendous backspin; instead he'd make the ball land pin high and spin sideways. It was all about finesse. Never try to squeeze extra yards out of the 8-iron through sand wedge. Instead, take one club more and swing easily. You'll get almost as much spin and a lot more control.
__Practice (carefully) on the course. __
Beating balls on the range has its limitations. It's hard to tell exactly how far the ball is flying, and it doesn't behave the same landing downrange as it does landing on a green. In my prime, I loved to go out at Wentworth and Lake Nona early in the morning before anyone got there and hit four balls into every green. I'd do this from the edge of the fairway or even from the light rough, so I didn't chew up the fairways, and I fixed my ball marks. I'd try for a slightly different ball flight with each shot. This "real" type of practice will sharpen your focus and benefit you in ways beating balls on the range cannot.
Try different follow-throughs.
The height of my ability to shape the ball was at the 1992 Open at Muirfield. I curved my shots both ways and altered my distances not by varying my backswing or downswing but by my follow-through. For example, a "chicken wing" finish, in which the left arm doesn't rotate but flies off your side, will force you to keep the face slightly open through impact, producing a fade. And it's fun.
Don't overdo ball position. If you want to hit the ball high or low by adjusting your ball position, don't move the ball back or forward by more than an inch. If you were to look down at address at silhouettes of three ball positions, the circles would overlap slightly. Move the ball any more than that, and you'll have to make a lot of complicated adjustments in your swing to compensate.
You can't excel with irons if you hit up on the ball, nor should you chop down into it at a steep angle. To discover the proper approach, hit shots with a 3-iron, the ball teed up a good two inches. If you chop down into the ball, you'll hit the top of the clubface; and if you hit up on it, your ball flight will be terrible. It will teach you a sweeping-type swing, the clubhead moving very slightly downward through impact.
Never swing with more than 80 percent effort.
My first teacher, Ian Connelly, gave me a lesson I've never forgotten. He had me hit six 7-iron shots as hard as I could. He then had me hit six 7-iron shots so easily they flew only 100 yards. He then asked me to increase the distance in 10-yard increments, six swings for each. Before I knew it, I was hitting the 7-iron the original maximum-swing distance, but with hardly any effort. It proved that good rhythm, not sheer power, is what makes the ball go long -- and straight.