DRILL: BATTLE OF THE BAND: (1) Get into an athletic setup, legs braced against the pull of the band. (2) Turn against the band, which keeps your hips from sliding. (3) Shift your lower body forward against the band's resistance.
In the golf swing, the body should do two things: rotate and shift weight. Those moves allow you to make a powerful coil and maintain it coming down. To coil, you have to turn above a braced lower body, which creates resistance.
Simulate that resistance by using a rubber exercise band. (I'm demonstrating above, but you can simply imagine the resistance the band provides.) Wrap the band around your waist, and have a friend pull it tight while standing to your left, on the target line. With the pull of the band, you need to be in a strong, athletic setup so you don't get dragged over. Then make your backswing turn, keeping your lower body braced over the ball. Feel your hips rotate and shift, not slide.
Then have your friend move to your right side, along the target line, and again pull the band tight. Mimic your downswing, feeling the band's resistance as you shift your lower body forward and your upper body responds. Your arms will drop into a good position.
Using a band is a great way to feel the swing's two power moves -- rotation and weight shift. Make sure they're a part of your normal swing.