Using a marker, draw a straight line on the side of your left hand and wrist where you can see it at address. The line should start at the big knuckle of your index finger and run over your wrist to about halfway up your forearm. That line should go from bent back slightly at setup to flat at impact. If it goes from straight to bent back, as I see so often, you'll never hit it solid.
2.) Attach a marker
To get a vivid visual of what happens when you achieve or fail to achieve this straight left wrist, use a marker attached to the back of a club (the marker's tip extending behind the heel) to trace the path of your clubhead along a piece of poster board. Hold the marker in place with a rubber band. A dry-erase marker and board work great for this part of the drill.
3.) Set up a plane board
The shaft angle of the club you're using determines the angle at which the board should be positioned in front of you. Take your setup, then prop up the board with a box or similar object so that it rests flush against the back of the shaft. For a driver, the board would be at a flatter angle; more upright for an iron. Make slow swings, and let the marker trace the club's path.
4.) Straight line = solid shots
Many amateurs try to drive the ball forward or lift it in the air. But good ball-striking comes from the club making a descending blow and compressing the ball against the turf, as shown above by the green route on the board. If you have the line on your left wrist straight at impact, the bottom of the swing is where it should be: on the target side of the ball.
5.) Bent line = weak shot
If you let your wrist bend backward through impact, which is common with players trying to help a shot into the air or add speed, the bottom of the swing moves back, before the ball, and you don't compress the ball into the turf. This is shown above by the red line. When the low point moves behind the ball like this, you hit fat or thin shots, or high, weak fades.
6.) Drag a towel to feel it
If you were moving a trailer, you'd attach it to the back of a truck and pull, not push. The golf swing works the same way. Wrap a clubhead in a wet towel and make some slow swings. The weight of the towel encourages the clubhead to come through after the hands. Some players think you have to add speed with the hands, but dragging multiplies the force.