There are obviously many aspects of coaching a tennis player. There are fundamentals, technique, footwork, strategy, sports psychology, etc. When a player is about to play a match, or in matches where coaching is allowed either during play or when players split sets, simple is best.
Many times you can help the player by getting them relaxed, excited about playing, and eager to show what they are capable of during a match. This means being very positive, possibly mentioning past successes, and really minimizing the thinking that tends to interfere with the body's ability to execute. I strongly believe that a player will perform best just using their instinct and habits during a point, especially if they have been practicing and preparing for the match or tournament.
The goal should be to get the same movement, swings, and shot selection during the point as a player tries to do in practice.
Reminding a player about one or two technique ideas is fine-keep it brief and simple and let the player go out there and play!