I think these are the 3 most common mistakes made when playing tennis!

1. Groundstrokes -- You play against hard hitters and you play very well against them! However, when you play against weak (pushers) players you over hit, get frustrated and lose. You probably play or practice against hard hitters. So, you have already made adjustments in your strokes for this. However, when you play weaker players, with slower shots, you think you should "kill the slow ball." However, you are doing this with a much larger backswing than you ever take against a harder ball. After doing this several times and failing by over hitting, you lose and actually start to play like the weaker player you are playing against!
    
   Correction: Learn to take the same size backswing on receiving a slow ball as you do when you receive a fast ball. On this slow ball, accelerate your racket head as you get nearer the hit. Keep the same size backswing!


2. Volleys -- Most players refuse to work on their volleys. They just get in the pattern only hitting or liking their groundstrokes. Typically, players like this never learn how to attack their serve and they play very defensive doubles.
    
   Correction: Warm-up first by hitting volleys. When you are just hitting, gravitate to the net for more volleys. They are an easy way to win a point. Learn the difference between an offensive volley and a defensive volley. The offensive volley might have a larger backswing than the defensive volley.


3. Serve -- You cannot seem to go to a higher level. You are not double faulting; you never win a point directly from hitting a serve. You are using the serve to just start the point, rather than trying to win the point.
    
   Correction: Go take a lesson, immediately! The new professionals have these things in common when hitting their serves. They are not holding the racket tight. They are lowering their tosses. Once they start their serve they keep the racket head moving. They do not start and stop the racket. They are getting more power from using their wrist, than from their shoulders.