ARMS DROP TO START THE TRANSITION
The most common mistake I see from amateurs is getting the upper body moving first in the transition rather than letting the arms drop before the upper body turns. This gets the club coming over the top and throws it outside the line so you swipe across the ball and hit a weak fade or slice. Try to wait at the top of the backswing and let your arms drop into position like you can see here. It requires patience in the transition and plenty of practice, but it allows you to really fire your body through the rest of the downswing so you can attack the ball from the inside with maximum club speed.