You are first going to want to focus on prevention and in this spirit; it means that you need to ensure that you are getting the rest you need between playing. This means there is no reason that you should go out and play a 4-hour tournament then turn around and practice for another 2 hours. No matter what you think, this is not a wise idea. Rather a better idea would be taking the day off after you have finished the tournament so that you can give your arm the rest that it needs.
Take some time to get into good physical shape. This includes ensuring that you strengthen your arms and back. If you ensure that your arms and back are in good physical condition you will have the muscle strength built up that will help you to prevent the injury. However, failing to make sure you are in good physical condition can make you exceptionally vulnerable to problems. If you have problems with getting into good muscular shape, you should talk to your instructor or coach if you have one. If you do not then turn to the internet to help you develop a good exercise routine that will effectively allow you to strengthen these essential muscles.
For those who do not play tennis for hours a day it is important to pace yourself. This means no playing for four weeks does not give you the right to run out and play for 4 hours straight suddenly. You should play for much shorter periods after you have had a long period of time away from the court. Your arm is abused in the process of playing tennis and suddenly jumping in with both feet will put you at risk of wearing it out badly. Instead, play much easier on your body and work to get back to the game that you prefer to play. Remember, pushing yourself to achieve the results that you want may seem like a great idea, but pushing too hard will leave you with an injury.
Always stretch before you start playing a game or even practice. In addition, you should stretch after you are done playing as well. This will allow you to keep the muscles surrounding the tendon as supple as possible. Often tense muscles can help factor into a tendon injury and will have you in pain quite quickly if you forget to stretch one day. A few minutes of stretching can go a very long way towards keeping you in good shape and injury free.
The last major consideration that you should take into account is you need to avoid continuously repetitive movements that drag on forever. If you are practicing a swing or a serve then that is wonderful but at the same time, you need to ensure that you stop on occasion to take a break. This will be quite helpful in working with you to avoid injury. If you are overworking your muscles without a break, you are putting yourself heavily at risk of an injury and you are not likely to enjoy the results. Taking care of your body is the primary thing that you need to be concerned with, not trying to stress over practicing your serve for another two hours straight. The time you spend on the court will be much more effective with a few small breaks sprinkled into the day.