One of the tips my father gave me was the importance of a good setup or address position. A great setup, apparently, is essential to a great swing.
That means, your body alignment has to be square to the line of the ball's flight. You have to be relaxed and perfectly balanced with your chin up (not in your chest) to leave room for your left shoulder to turn in the backswing. This will help to keep your ball go straight. You have to have your back straight and be bent forward from the hips. Your knees are bent but not rigid. Ball position is essential: a ball too far back may result in the ball going to the right; a ball too far forward may result in the ball going to the left. Similarily, the width of your stance is also important: a width too far apart will restrict your backswing, while a narrow width will affect your balance. Of course ball position and width of stance are both influenced by the club used. So it's important to memorize these for each of your woods and irons. Another great tip is the power of a preshot routine, but I'll get into that later!!