Many golfers overlook their posture. Rather than take the time to see that their weight is properly distributed at address, or that they have the right amount of forward tilt, they set up to the ball quickly and carelessly and simply let it fly.
If this is your preshot pattern, slow down! You need to pay more attention to your posture. If you don't, you'll have a hard time maintaining your balance and keeping the club on the desired swing path. On the other hand, the better your posture is, the more likely you are to move everything properly during the swing, and thus build clubhead speed and power.
Good posture is grounded in a forward tilt from the hips -- not the waist -- of about 30 degrees. If you're not sure how much 30 degrees is, picture yourself standing in the middle of a clockface. If your upper torso is the top hand, it should point toward one o'clock. Think of this as your swing's "Happy Hour:" You should maintain it for as long as you can, from your backswing to your finish. This posture prevents you from moving up or down during the swing, and helps guide the club on the correct path, which is at 90 degrees to your spine.
Why 90 degrees? If you were to attach a ball to a rope and swing it around in a circle, it would move fastest when it's rotating at 90 degrees to your arm. The golf swing works by the same principle: For the club to move its fastest, it must rotate at 90 degrees to its axis, which, in this instance, is the spine.
When your address is too erect (12 o'clock posture) or bent over (two o'clock posture), it's almost impossible to swing the club on this path. When vertical, the tendency is to roll the arms early during the takeaway, promoting a very round swing path. The likely result is a pull and maybe even a topped shot. When hunched over, the tendency is to swing the club too much from the inside. This makes it difficult to clear the left side of the body out of the way during the downswing, leading to an open clubface, and, in most cases, a shot pushed to the right.
At address, picture yourself standing on a balance beam. To prevent yourself from falling off, you have to balance out these joints. Leaning too far back (on your heels) or too far forward (on your toes) will tip you off the beam.