Improvements in sport can be broken down into three aspects: Physical, mental and technical. The order of importance of these aspects is often misconstrued. Technical improvements come down to things such as shot selection: When it is appropriate to play that shot. Here are some key areas of advice that, once practiced so that they are embedded in your game, will bring drastic changes to your results:
1) Imagine that there is a washing line two feet above the net and never hit under it. You should always allow room for error with height and spin over the net. This goes for aiming at the line as well: Never do it. Always look to place the ball with a large enough margin for error. Aggassi used to say that he never tried to hit a winner, he only tried to place the ball where is opponent would give him an easier next ball.
2) Once you are on the attack, remain on the attack. Do not hit an aggressive shot and stand further back in the court, prepare yourself to move forward.
3) If you are struggling in a rally, just think I am going to try and make my opponent hit one more ball every time to are moving to play your shot.
For a player over the age of about 12, I'm sorry to say, is not going to be able to improve their technique drastically. This is simply because it is extremely hard to "un-learn" a certain technique that you have practiced so profusely previously. The best that you are going to be able to do is compensate with this technique in order to use it to the best of your abilities.
However, for those thinking of giving up on their desired tennis improvements, do not loose faith. There is a great magnitude of things that you can improve that will raise the level of your game drastically. The physical aspect of tennis is the second most important element that you can improve to benefit your overall tennis capabilities. To do this you will first need to build up a strong aerobic base. This can be done by extended jogging with your heart rate at about 170, with intervals of 180. You should aim to jog for a 45 minute period, and within this period jog for two five minute sets at a heart rate of 180. After doing this three times a week for a month you will be ready to engage in more tennis specific fitness. This will consist of sprints. You should be running sprints at a work to rest ratio of 3:1. So if you sprint for ten seconds, you rest for thirty seconds before you start again. These sprints should be incorporated into a regime of swimming and skipping. This will cover the endurance side of fitness. You should also enter a weight training programme, but remember being "bulky" is not the ideal physique for a tennis player; You should be looking for lean muscle. Also the greatest muscle that can help your strokes is the rotator cuff muscle in your shoulder. Use any search engine and type in exercises for this muscle in order to get a regime that will give you more control over you shots.
The mental aspect of tennis is the separating factor between being top five hundred in the world and being number one. The winner of Wimbledon, or the US Open, will be the player who is able to control his emotions on the court throughout the two weeks needed to win. Federer was a great example of this when he won his first Wimbledon title. Watching him he looked as if he didn't even care about what was going on around him, he was so focused. He showed little emotion throughout the whole tournament. But then at the end, when he won in the final, he fell to the floor and let all of these emotions out. Had he been doing this throughout the whole two weeks, he would not have won the title. To improve your mental ability on a tennis court, a huge tool to use is visualization. This is where, before a point you visualize what you are going to do in your head: You see yourself serving the perfect serve that is the ace. So your brain now believes that your body has completed this action before and is more likely to be able to produce the electrical signals in the nerves in order to be able to do it again. This can also be used in practice to try and perfect your shots and give you confidence in them. Confidence itself is one of the most important things that can change your game for the better over night. If you get in better shape and follow simple game plans that mean you are going to make less unforced errors, as explained earlier, then this is going to give you more confidence and improve your game extremely quickly.
So to improve your tennis game, you should not be focusing on your technique as much as many may tell you. Playing college tennis to a high standard myself, I find myself working on technique less and less and I find myself in the gym more and more. In practice all i think about is taking the ball early and moving through the ball instead of lifting up when I strike it. I don't worry about my technique because this is not going to change a great deal. Therefore focus on the aspects of your game that can change and will help you improve your tennis game.