This story is for you if...
1. You don't feel confident about your backswing.
2. You tend to swing over the top.
To get your swing on plane you need to groove a solid position at the top of your backswing. You can practice this a dozen different ways, but here's the easiest.
As you make your backswing, slide your right hand to the bottom of the grip. Notice how this causes your right elbow to fold naturally and your left arm to stay straight and stretch across your chest instead of lifting up, which is a common result of an exaggerated inside move in the takeaway.
After a number of repetitions you should start to feel more comfortable at the top of your swing, and you'll notice a different sensation as you start your move down. Try swinging with your normal grip immediately after repeating this drill several times and the sensation should stay with you. Continue alternating between the drill and regular swings until they feel similar. You should quickly see the shape of your shots change from a slice to a straight ball, or even to a bit of a draw.