Maintain a firm grip on the club at the top of the backswing. Do not open up the hands as this will lead to an excessively long backswing and a slower downswing. This tip may appear to be elementary, but you would be surprised to see the number of golfers, including high-level golfers, that do this.
But by maintaining a firm but relaxed grip at the top you will be able to start the downswing faster and with more force (see Exer Rings for affective grip development). This will give you not only greater distance but also accuracy.