Among the most common mistakes that slicers make is taking the club back on an overly vertical plane. This makes the left arm fly too high in the backswing, effectively "disconnecting" it from the body. The only way to get the club back down to square at impact is to swing across the target line, which tends to yield high, weak banana balls. There's a better way!
To shallow out your downswing path, set up in your normal address position without a club and turn back as though you were going to high-five someone directly behind you. To maintain your original spine angle and execute the high five, reach your left arm across your chest (at shoulder height), not up toward the sky. This keeps the left arm tight to your chest, resulting in a more on-plane backswing and better contact. (See photo above.)
Grab a club and repeat the same move. When viewed from behind, your left arm should be on the same angle as your shoulders, with the elbow pointing just outside your target line and your left hand even with or just slightly above your right shoulder. That's an on-plane backswing. What you don't want to see is your left hand and arm above your right shoulder, with your elbow pointing inside the target line. The left arm should swing back on the same line as your shoulders and virtually "cover" both shoulders at the top of your backswing. Get this right, and you'll have better control and distance.