Despite all the talk you hear about increasing power in a golf swing, it's increased club speed that really increases your distance. Specifically, you're after increased clubhead speed. You need to understand the difference because you can make the clubhead move faster even though your swing might appear to be slower.
Learn this fact: Gravity is enough to help you develop clubhead speed. Several years ago, Dr. David Williams conducted a research project on Bobby Jones. He discovered, through a computer analysis of some swing footage that Jones made in the 1930s, that Jones had a swing speed of approximately 113 mph. That's in line with modern pros. Jones accelerated the club during his downswing at just more than 34 feet per second per second, a figure that is slightly faster than gravity alone (32.2 feet per second per second). Once you realize that you don't need extreme body speed to create clubhead speed, you'll stop working against yourself.
Watch good players swing. Spend time watching good players and imagining how their swings must feel will help you ingrain the rhythm for your own swing. Ernie Els is one such player to watch; that's why he's called "The Big Easy."
Relax your body, especially your arms. Relaxed muscles move more quickly than tight muscles; this simple fact has been taught in the martial arts for centuries. When you tense your muscles, you interfere with their natural movements. Relaxed wrists cock and uncock more readily and at the right moment. Relaxed legs and arms don't jerk the club from its natural plane of movement. A relaxed torso will make a fuller turn with less effort.
Focus on swing rhythm. Many instructors talk about swings as if they were pendulums. While the motion isn't exactly the same, the image of a long, flowing swing is a good one. Too many players swing the club back too fast, and then jerk the club down from the top. The downswing is slightly faster than the backswing, but relaxed muscles automatically dictate this rhythm. If you try to swing too fast, you'll have to tighten your forearm muscles a lot at the change of direction. If you try to swing too slowly, your swing will be jerky rather than smooth.
Practice swinging as quickly as you can while you stay relaxed and keep your rhythm. Don't be surprised if this swing feels very much like your practice swing, only faster. You'll find that your swing problems will lessen or disappear because many are the result of trying to "kill the ball."