A large percentage of unforced errors are caused by poor foot work. Usually, the typical player moves to the ball too slow and then moves away after making the shot too fast.
 
The fix for this is to move to the general area of the where you will hit the ball as quickly as possible. But, do not plant your feet too soon! This causes a lot of mishits. Try to not plant your feet until after the bounce (reading the speed and spin of the ball better). Try to be still during the stroke (watch the ball and keep your head still).
 
After you hit your shot, a big mistake is to leave the shot too soon, trying to recover too quickly. This movement right when you meet the ball can cause a huge number of errors. Most players are taught to shuffle back to their original position. However, the key to this is to try to regain some of the court you lost by moving to the ball.
 
The general rule is to shuffle back to your ideal position or stop wherever you are when your opponent strikes the ball. But, do whichever one happens first! Better to be still and not move exactly where you think you should be until you read the exact direction of the oncoming ball. Then you will be moving 100% in the right direction. This is better than when you recover so fast after hitting the ball that your opponent always hits behind you. This is very inefficient and exhausting.
 
Remember, be still when you are hitting the ball (especially keep your head still) and on recovery from making a shot. Try to get still when your opponent hits the ball, even if you are not exactly where you think you should be in recovery.
 
Good luck on the courts!