Working on keeping your head behind the ball can help cure a lot of golf swing ills. If your head gets ahead of the ball at impact, it's likely that you will hit a high slice or a smothering pull. You will not be able to get the distance you should be getting if your head is moving forward excessively during the forward part of the golf swing.
Start with Putting
When you're at the club or the practice range, put the driver away and head for the putting green. Hit some putts from about 20 feet for about 20 minutes, focusing on keeping your head still. Get the feeling of your shoulders moving your arms with your head behind the ball. Keep your weight even on both feet during the putt and don't worry about whether or not the putts go in. You are simply trying to develop a new feeling.
Practice Your Chipping and Putting
You're not ready to go the driving range just yet. Instead, grab a pitching wedge and hit some chips, then some pitches. Remember that you're trying to focus on keeping your head still laterally and vertically. You can only hit solid chips and pitches if your head is almost totally still; as soon as you keep your head still for these shots, you'll feel your arms moving forward and through without your body lunging forward. Remember to keep your weight evenly balanced.
Learn the Feet Together Drill
Now it's time to hit some full shots. Take an 8-iron and put the ball on a tee for this drill. Make some practice swings with your feet together, about three inches apart at the most. Don't worry about hitting the ball a long way as it's important right now to make smooth swings with your head behind the ball at impact. In this drill, if your head is in front of the ball at impact, you will fall over. After hitting about 40 balls with this drill, get the driver out and make some full swings.
If you are hitting a bad slice or some smothering pulls, the problem may well be that you are lunging at the ball and not keeping your head behind the ball. However, it's likely you are making some other mistakes, probably with your grip and alignment. Keeping your head behind the ball at impact, especially with your driver, will take the really poor swings out of your arsenal. If you need more advice, make sure you see your PGA or LPGA teaching professional.