A common cause of tension lies in the grip. An improper grip, or one that is too tight,
creates tension that continues up the arms, through the shoulders, down the body and to
the legs is sure to wreck your swing.
Tension is the greatest robber of motion. The result is a hitting action rather than
a swinging motion. A correct grip can relieve the pressure; however, it's still
possible to have a correct grip and hold on too tightly (tension).
Bad posture and set-up is another cause of tension. By playing the ball too far
from the body, the player reaches out and this encourages the weight to move too
far forward on the feet. The result is an off-balanced, blocking motion.
For good posture, the feet should not be too far apart (about the width of the
shoulders), with the weight evenly distributed and to the inside. Knees are
flexed, back is straight and tilted forward from the hips. Let your arms
hang naturally with the right forearm under the left forearm and the right
elbow relaxed and pointing downward.
The shoulders set up the planeswing, therefore they should make a good full
turn. A limited shoulder turn leads to a short, fast backswing which destroys
good tempo. Don't start something you can't finish.
After completing the shoulder turn, the shoulders become passive on the
downswing. This means that the forward motion of your swing is lead by the
lower body. The knees, moving forward and towards the target, starts the
downswing and eliminates the tension of the upper body wanting to take
over the downswing. This is commonly called "coming over the top."
Food for thought: To quote a famous golf pro friend of mine, he once said,
"Swing the club as though you were working by the hour, not by the job."
This would most certainly help lead you towards good tempo in your swing.
So develop a tension-free swing for maximum distance and accuracy.