Making an arm swing that's out of sync with the body turn causes all sorts of problems. Here's a simple drill to learn the feel and technique for maintaining this crucial relationship between the arms and chest throughout the swing.
1. Tie a loop on one end of a string and affix the loop to your left pinkie (left, inset). Place your left hand on the grip, running the string between the grip and the inside of your hand.
2. At address, use the right hand to draw the string up to the second button from the top of your shirt (left), which in most cases will be your sternum.
3. Swing to the top of the back-swing with your left arm, keeping your right hand on your chest and the string in place. The string will remain taut if you swing correctly.
You can do another version of this drill by starting from address and, without any backward motion, swinging forward to the finish while keeping the string straight.