The shot you hate
Most of your long-iron shots start low and left, then slice wildly to the right.
The causes
Three things combine to produce this ugly shot: Your downswing is too steep, your swing path is too outside-in, and your clubface is open. Chances are also good that your chest and left knee face the target at impact, which in turn pushes your left hip back so that it's inside (and farther away from the target than) your left foot.
The cure
Have a friend stand opposite the ball from you at address, then swing your club to the top. Start a slow-motion downswing and shift your hips toward the target so that your left hip is over your left foot, and then very slowly bring the club back down until the shaft is just below your waist. Have your friend check your movements to make sure they resemble the bottom photo, not the top. Otherwise, you're in full pull-slice mode.

Chest Ahead
If your chest points ahead of the ball at this point in the swing, you're bound to cut across the ball and pull-slice it.
Sad Twist
If you simply twist your hips from the top your left hip will move away from the target. It should move toward the target.
A Bad Break
Solid long-iron shots need a solid left side, and if your left knee bows out like this you're in serious trouble.

Chest Back
Shift more and twist less to stack your left hip on top of your left knee and ankle.
Hip Forward
Keep your chest pointing behind the ball to promote the proper inside-out swing path and a straighter ball flight.
A Solid Knee
If your knee remains stable like it should, it will point away from you-not at the target-all the way until impact.