The Cure for a Fat Shot
This problem may be easier to fix than you think. You may not realize it, but your back shoulder may be dipping towards the ground. This move forces the club to hit the ground too early. Most of the time it happens when you are really trying to get into the ball. Imagine? You're 200 yards out and you've chosen to hit the 4 iron. When you've got it in your head that you have to hit the ball hard to get it there, it's natural tendency to start your downswing with the hips in an effort to "really get into the ball". That may work in baseball, but not in golf.
I'd like to take you through what happens if you make that move in golf. First, get in the your setup position, now take your backswing and hold it at the top. Now move only your hips horizontally towards the target, and notice the way it forces your back shoulder to "dip". If you've made that "dip" with your back shoulder, it's over. You are going to hit behind the ball 90% of the time, and if you do make good contact, you'll probably end up with a killer slice. Lateral movement is no good for the golf swing.
To avoid this problem, and to simplify your golf swing, keep the lower body out of the equation. If you really want to get into the ball, start your downswing with your arms. It's tough to do if you have a habit formed, but once you get used to it, you won't be hitting the ball fat anymore. Most golfers over-emphasize the weight shift. If you keep your hips still, the weight shift will still occur, but it will happen naturally.
Get to the top, keeping your leading arm as straight as possible. Now start down at the ball with your leading arm fully extended. Your head hasn't moved laterally, and your hips haven't moved laterally. Now to take this to the next step, imagine the entire golf swing, through impact, occurring underneath the upper body.
Concentrate on keeping your front shoulder strong and "down" on the ball. This will enable you to keep your swing on the correct plane, and it will help your hands to keep up with your body. Don't let that shoulder come up and away from the target before impact as that will cause your body to get ahead of your hands leading to fat shots and slices.