Serve and Wander
It is not uncommon to see beginning, novice and even intermediate players lose track of where they are after they hit their serve. Here is how to identify and solve this problem.
Some players will be attracted to the ball they just hit as though it is a magnet drawing them in. While players may get away with this type of recovery if the return depth is short or a medium distance, a deep return will cause trouble.
Two Kinds of Problems
An in-court recovery means that a deep return will either volleyed unintentionally (breaking the two-bounce rule) or will lead to the ball landing at the server's feet. This will make it virtually impossible to hit a quality third shot.
Rather than telling player to 'stay back', use the more active instruction 'move back'. This implies that the player must do something proactive after they deliver their serve. Ideally, the player is about half a step behind the baseline when the return is struck.