By far the most common problem average players have is slicing. Their shots start to the left of the target and curve big time to the right. That's because they swing the club from out to in, cutting across the ball with an open clubface. It's the classic "over-the-top" move, and it produces those weak shots to the right that so many golfers are trying to eliminate.
Just about all better players swing into impact from inside the target line. To learn to consistently swing from the inside, put a large sponge on the ground a couple of inches to the outside of the ball, on the backswing side. The sponge acts as a visual reminder: It's telling you, Don't go there. Using a middle iron, practice hitting balls without hitting the sponge. If you can do that, you'll be coming through on a great swing path (pictured). Make a mistake and clip the sponge, and you'll get instant feedback, without hurting your hands or your club.