To prevent tennis elbow
, strengthen the forearm extensors by doing eccentric, aka "negative", exercises.
This involves slowly lowering your wrist while the forearm is supported on your thigh. Begin by using a small dumbbell or similar weight. 1-5 lbs. Is usually adequate. 2 sets of 10-20 reps every other day. Also, avoid repetitive wrist extension and gripping, especially with heavy weights.