I've always liked Jim McLean because he tries to simplify the golf swing for us amateurs. He is the father of the 8-step swing, where he breaks down the swing into 8 simple steps, and the idea of the X-factor, which is where the shoulders out-turn the hips by a small amount creating an "X" from a top down view.
This tip is regarding the top of the back swing, where the X-factor comes into play.
-----The text below is directly from the Jim McLean website----
I'm partly responsible for the common swing flaw you see me demonstrating below, which stems from an incorrect application of the X-Factor swing theory. As I first documented 10 years ago, the X-Factor -- the difference in how much the shoulders turn versus the hips -- can be a tremendous source of power. The problem is, in an effort to increase the X-Factor differential, many golfers restrict the hip turn too much. The result is less power and consistency, not more.
You want to create resistance in your lower body as your upper body turns behind the ball, but not at the expense of a proper hip turn and weight shift. Your hips should turn rotationally and shift laterally slightly as you swing back. Achieve this by moving the left knee back behind the ball, rolling off the left instep or picking up the left heel. You can also flare out your right toe at address to increase your hip turn.
Set up normally to a teed-up ball, using a mid-iron.
Slide your left foot back toward the right, so that they practically touch each other, and the clubhead is about a foot behind the ball.
Start the backswing. When the club reaches waist level, step forward with your left foot, returning it to its original position, like a batter stepping into a pitch.
The clubhead will still be moving back as the lower body moves forward, which automatically increases wrist work and prevents the right shoulder from leading the downswing.
Assume your normal position. Then widen your stance by placing your right foot well outside your right shoulder. Fan the right foot outward, so it points away from you at a 45-degree angle.
Hitting practice shots from this position helps you eliminate any upper body slide. It maintains a wide gap between your knees by dramatically slowing the right leg action. It also keeps you from spinning the lower body too fast.