To hit the ball harder athletes are advised to undergo a strength training program. This of course is good advice but it is limited. Many studies have shown that after 8 to 12 weeks of a strength training program results begin to diminish.
In addition, the more strength training that is done, the more explosive strength decreases. This in turn is a negative for performance on the field or court where speed and power are more important than pure strength.
Rather than only doing strength training you should instead do some technique work to learn and master the actions that produce the power in hitting. This usually involves a simple change in technique that allows you to hit the ball with much greater force than you typically exhibit.
More specifically this means must learn to shift and rotate the hips prior to involving the shoulders and arms. Strengthening the hip muscles exactly as they are involved in these actions will increase your force even more so.
For more information on learning and perfecting these exercises See Explosive Tennis: The Forehand and Backhand and Biomechanics and Kinesiology of Exercise.