Racket: Too heavy a racket will overload the muscles of the forearm, however, too light a racket and will cause too much vibration at impact
Tennis Balls: Use pressurised tennis balls as these are softer and absorb more of the vibration at impact.
Strings: Try switching to a multifilament string as they will absorb more of the impact compared to a polyester based string.
Tension: Try reducing your string tension to below 50lbs. Lower string tension allows the ball to stay on the strings for longer reducing peak impact forces acting on the elbow.
Footwear: Tennis players shoe should be broad based with a solid heel counter and adequate grip on the sole of the shoe to assist the player when changing direction on the court. Running shoes for example are not adequate for tennis as they are too narrow fitting and are purpose built for running in a straight line not changing direction at speed. If worn for tennis they will increase the likelihood of an ankle sprain.
Grip Size: If the grip is too small the player has to grip the racket harder to stabilise on impact, this greater workload on the muscles can predispose to injury.