Practice everything, but focus on strengths and weaknesses
One thing I found important when practicing or coaching became almost a mantra for me. The mantra was, "Practice everything, but focus on strengths and weaknesses." The idea was to develop overpowering strengths that you can dominate with, while getting rid of any weaknesses.
Some players tend to focus on their weaknesses, often getting so overly critical that it's all they think about. They forget that matches are usually won by a player dominating on something. You can't do that unless you develop something to dominate with, and then develop your game around it. In particular, focus on developing both that strength and the shots that set it up, especially serve & receive.
At the other extreme are players who get into the habit of doing the same drills all the time, session after session, and so they get good at the things they are used to practicing, but never get around to fixing the problems in their games. I once saw a player with a great forehand counterloop lose a match because he couldn't block on his backhand side. Later he had to play the same player again. How did he warm up for the match? Rather than have someone loop to his backhand, he spent about fifteen minutes forehand counterlooping with someone, then went out and lost again because he again kept missing backhand loops. Then, at practice the next day, he spent half the session counterlooping again, and never got around to working on that backhand block.
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