One of the most common faults I see with weekend players is in the transition from backswing to forward swing. Watch any tour player's swing in slow motion and you will notice that their change of direction from backswing to forward swing begins from the ground up with their feet and knees. Most amateurs however start the forward swing with their upper bodies either by heaving their shoulders or a jerky attempt to force the club towards the ball with their hands. Both movements result in a significant loss of power, poor contact with the ball and misdirected shots.
To help my students feel the proper transition movement I use two drills. The first is to have the student go back to his baseball or softball days. I'll have them pretend they're at the plate and swing as they did when hitting a baseball. They naturally begin their swing into the ball from the ground up using their feet and knees. It's fun to watch their expressions when the "light bulb goes off" as they sense the movement and then relate it to their golf swing.
The second drill is to have my student swing the club to the top of their backswing then hold that position for four or five seconds. This allows them to feel their feet and legs resisting the coiling action of their upper body. The deliberate pause also enables the student to trigger the downswing as a reflex uncoiling action with their feet and legs.