In order to achieve your highest available swing speed and therefore power, it is essential to make the correct backswing.
One of the keys to creating power is to maximize the weight transfer through the ball. This can only be achieved if the backswing transfers weight onto the back foot (foot furthest from the target).
The trick is to maintain good posture and concentrate on lining up the left shoulder directly over the right knee (reverse for left handed players). This will ensure a significant amount of weight has been shifted to the back foot and thus loaded up properly for a powerful golf swing as the weight is transferred onto the front foot during the downswing and follow through.