There are so many different elements that effect your tennis game, it's something you can work on your entire life. One thing that many recreational and competitive players need to improve is the quality of your practices and workouts. It's so easy to walk onto the court and exchange groundstrokes with your hitting partner, not really thinking of working on any particular aspect of your game. If you instead practice with a purpose, you'll find it has a significant impact on your ability to raise the level of your game when it counts, in a real match.
Some tennis players like to practice by running drills and focus on intensive stroke production, while others practice by playing a social match. Neither player is right or wrong because it's an individual preference, one that you need to find out for yourself what works for you. In either case, you need to give it your best effort and make the most of your time on the court. Here are some tips on getting the most from your tennis workout, and hopefully improving your game.
When setting up practice matches, try to find a variety of players to hit against. If you like hard hitting and fast action, don't neglect to play an occasional match against "pushers" and players who don't hit much pace. The only way you can beat more players is to develop a game plan against them, and the only way to do that is to play more matches. If your tendency is to play practice matches against the same group of people, make an effort to look outside the group to a player who can challenge you in other ways. If you're faced with the same style game every time you play, you won't be forced to develop other aspects of your game. Try to play half of your practice matches against people of equal ability as you, where either can win on any given day. The rest of your matches, the other 50% should be split half against players better than you and half against players worse than you. This way, you get a variety of matches and playing styles to help you improve your strategies and work on different strokes.
This is one of the best ways to work on stroke production, ball placement and strategy. It doesn't involve playing a real match, though you may want to incorporate practice points or games into the workout. You can drill using a ball machine, a hitting partner, with several other people or alone with a large basket of balls. The idea of doing drills is to work on specific aspects of your game or strategy. For example, in doubles it's important to get the first serve in and make the first volley, so you may want to practice your serve-volley while your partner practices crosscourt returns. You can also divide the court into quadrants and play "target practice" where you try to hit specific areas on the court (it isn't as easy as it sounds). Again for doubles players, you may want to focus on hitting your shots into the alley, or deep in the court. If you want to work specifically on your forehand or volley (or other stroke), it may be best to rent a ball machine for an hour. This type of practice will allow you to focus on just one thing, and get grooved in so that you're ready with that shot for your next match. Lastly, something most of us don't think to do, is grab a large basket of balls and hit serves. Not just "hit" serves, but see if you can get 8 of 10 up the tee, and work on getting your second serve deep. Your shoulder may be sore after this exercise, but you'll be amazed how easy it will be to hit these serves in your next match.
The other part of tennis that sometimes gets ignored but is so important in big matches. Conditioning may alone be the determining factor in some tennis matches, depending on length of match and court conditions (i.e. heat, humidity, etc). The key to conditioning is not just running, though endurance is important, but also up-back and side-side movement as well as stopping changing directions. Short of running a few miles on the treadmill, you can use on-court routines to improve your conditioning as well. Have your partner feed you balls and practice running in to hit a short ball, then immediately run back for a lob. For doubles teams, you can work on crossing over and poaching on a short ball and covering your partner at the baseline. There are also games which don't involve hitting the ball at all, affectionately known as "running lines" - starting at the corner of the court, run up to the net, the back-pedal to the service line, run to the middle of the court, then up the middle line to the net, back to the service line and over to the other side of the court, up to the net, then back-pedal to the baseline and side-step back to where you started. You can have you partner mirroring your moves on the other side of the net and make it a race. Another drill to improve movement on court is to place tennis balls across the court, then run to the first and pick it up, run back to where you started, then run to the second and pick it up, etc. When you have picked up all the balls, reverse the drill and place the balls on the court where you found them.
All these drills and games are designed to improve your enjoyment of tennis by making it easier to play. When going out to your next tennis practice, try to give it 110% and see how you feel. It's amazing how great a good workout can make you feel about yourself and about your tennis game.
Have fun on the courts!