Tennis elbow is usually caused by gripping activities, and gripping either too hard or for too long can bring on the pain. Make sure the item that you are gripping, whether it's a tennis racquet, a hammer, or a canoe paddle, is the correct size for your hand. If it is too small it will cause you to grip too hard. If you play tennis for the first time in a long while, or you have to do a strenuos activity such as decorate a room in one weekend, make sure you take regular breaks and stretch the muscles which work over the wrist by doing 'limp wrist' and 'policeman halting traffic' type stretches.
For those who have suffered from tennis elbow in the past it may be a good idea to wear a Tennis Elbow Support. They work by preventing the wrist extensor muscles (that run along the outer side of the forearm) from contracting fully, thus reducing the strain on the elbow.