The following are a few simple tips that can truly help any level of play. My tips are simple and can be applied with or without a coach. 

Whatever success I have had is a direct result of seeing and then saying a few simple words. Demonstrations are also very important because so many people react to vision learning a lot quicker. 

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Tip 1: Early Preparation

You cannot prepare soon enough- turn your hips and shoulders before the oncoming ball bounces on your side. GET THAT RACQUET BACK! 

Tip 2: Exaggerate Your Follow Through

The majority of all people, no matter what sport they play, can improve by following through when you get a little nervous or your opponent is in control of play and moving you around.

Exaggerate your follow through; do not think just hit the ball. MOVE YOUR FEET!

Tip 3: Two Bounces and You’re Out

Whenever you rally, wherever you play, make up your mind that you will never let the ball bounce twice. Always run for the ball even if you think you have no chance to reach it.

Richard Williams told Venus and Serena when they were six and seven years old, “Don’t think, just run and hit the ball.”

Try playing on a court with no lines, whatever comes to you, it is in play. 

Tip 4: Hit-Recover

So many players will hit a ball and watch the ball and compliment themselves on the shot. From now on you must hit and recover. 

I suggest you watch the pros- it actually looks like the pros are recovering in the middle of their hit. I don’t suggest going to that extreme. Just hit, finish and get back to position.

Tip 5: Foundation

I want you and your practice partner to start at the serve line but stand in between the singles and doubles lines. Rally or play points hitting only between the lines. 

As you gain confidence, move back further until you are on the baseline. It’s critical that you have a solid foundation and complete balance in order to hit between the lines (also early preparation).

Tip 6: A Safety Target 

So many players will overplay the ball, including playing the lines. 

Vegas spends billions of dollars building casinos. Why? The majority of gamblers do not play the odds, they go for it and they lose!
More: 4 Game-Changing Defensive Tips

This is exactly what tennis players do; they defy the odds and go for winners. Watch how the pros build points, they play offensive but do not go for just winners. 

Tip 7: Boris Becker (He Had 4 Serves)

When I worked with Boris he taught me so many little things that helped me become a better coach. This is how he approached his serve:

Got into his ready position;
Went through his entire serve motion including where he would serve to;
He would do his 1st serve and repeat if he happened to miss his 1st.
More: How to Improve Your Second Serve

This ritual slowed him down and instilled in his mind what he would do when serving the ball.

Tip 8: When Do You Communicate?

There are so many opinions on when you talk, how you talk, and what you say to your player after a practice session or a match.

There are no two people alike, you have to adjust your method to fit the student’s personality

Source:  http://www.active.com/tennis/articles/8-coach-s-tips-to-improve-your-tennis-game