I hate to start out with a negative, but I really have no choice. Far too many amateurs make the same 500-pound-gorilla of a mistake when they try to crack a monster drive: They muscle the ball. Figuring that big distance requires big effort, they tense their shoulders and arms, hurry their transition and make a furious lunge at the ball with the top and bottom halves of their bodies simultaneously. Even when they manage to hit a straight drive this way (rare), the ball doesn't go nearly as far as it could.
Momentum, not force, is the key to building the clubhead speed you need for booming drives. With a proper backswing you'll have lots of stored-up power at the top, which will allow you to accelerate the clubhead easily and naturally—to pour on the momentum—on the way back down.
If you want to understand the difference between force and momentum, watch the Tour's long hitters. Ernie Els may be the best example. When I play with Ernie he routinely hits the ball twenty yards past me. Then, all of a sudden, he hits it sixty yards past me with what appears to be the same unforced swing. How?He may widen his stance a bit, but mainly he focuses on staying even more stable than usual with the lower body and holding off his transition a smidge longer at the top. He stays smooth and in control on the way down, building up his amazing clubhead speed through good timing, good mechanics and an uninhibited release. He doesn't try to swing harder, only better, so that he can generate more momentum.
Learning to rely on momentum rather than force requires mental toughness. The best single piece of driving advice I can give is to strive for freedom as you swing, but that's not easy. It helps, of course, to have confidence in your fundamentals, particularly in standing tall at address, anchoring the backswing with a flexed, rock-solid right knee and resisting the coil of the upper body with the hips and legs. Confidence frees you to release the club through the ball with full acceleration and to feel as if you keep accelerating even after impact, down the line with the right shoulder, and through to a high finish (see "Wrap It!" sidebar). But to fully trust in the power of a free-flowing swing, you must first prove to yourself how easily a good downswing generates momentum. The Twenty-Yards-Farther Drill is the best I know of for teaching that, even if it uses a six-iron instead of a driver.
One reason so many amateurs find it hard to resist whaling away at a drive is that they have bad practice habits. At the range, they'll hit mostly drivers while swinging as hard as they can. This is always counterproductive, but never more so than when, as inevitably happens, they manage to hit a few drives on the screws that way. The next time they get to the course, the feeling of those accidental winners will be all they remember and what they will try to re-create. Watch the pros practice at any Tour event and you will see that they seldom hit more than a handful of drivers in a row before switching to an iron to slow down their rhythm. I personally like to mix in a wedge shot after every few swings with a driver, as a quick way to reclaim the leisurely feeling of proper momentum.
If you are serious about improving off the tee, take the time to identify your maximum, controllable swing speed and try to discipline yourself to stick with that speed whenever you play. To find your maximum swing speed, hit a few easy shots with the longest iron you can swing consistently well. Then grab your driver and make exactly one swing, trying your best to imitate the rhythm and momentum of that controlled long-iron swing. Hit three or four more iron shots, then make another single swing with your driver. Memorize the feeling of these easy swings. To further hone this sense of controlled rhythm with a driver, have some fun working on the High-Tee Drill.
Transporting your smooth new driving rhythm from the range to the course is another challenge, but the key is to pay more attention to your practice swings. When first getting familiar with your maximum swing speed, take your practice swings with a long iron before drives (and anytime you feel things speeding up). Another useful pre-drive routine is to find some rough near the tee and make a strong swing through the tall grass to get a feel for the resistance. With this or any practice swing, make it as real as possible. Zero in on a small, very specific target, make an easy backswing and then a full, free-flowing release. Hold the finish while staring at the target. And remember to keep that right knee flexed, because straightening it is often the culprit when drives break down. At all costs, guard against getting too tight and trying to steer your drives. Instead of focusing on hitting the ball itself, focus on finishing the swing. Feel as if you accelerate through impact. The very best swings are those in which the ball just seems to get in the way of the clubhead.
With time you will come to associate an easy, rhythmic swing with power and confidence. If your best drives don't routinely go as far as you know they possibly could, that's a sign you're playing smart golf. I guarantee that the guys who are winning your club championship are not trying to hit monster drives. They are trying to hit controlled, moderately long drives into the fairway, and that's a great strategy.