While backing away from the net makes sense if your opponents are hitting an overhead smash or another incredibly fast ball, in most situations it causes more problems than it solves. Here's why it happens and what you can do about it.
Identify the Problem
Very often players aren't even aware that they are retreating. Begin by helping your players see what it is they are doing. You can stop play in the middle of the rally and show them how far they have moved off the line.
Understand the Rationale
Ask your players why they think they are backing up. In most cases, it will be because backing up gives them more time to react (this is especially important if they are fearful when at the net). Help them to understand that while this may be true, the increased distance also gives their opponents more time to react, and makes it more likely they receive uncomfortable balls down at their feet.
Develop Volley Skills
Your players will be much more comfortable holding their ground if they have faith in their volleys. Help them develop the reflexes and the technical skills to play these shots with confidence.
And dropping a few pylons behind the NVZ can help players to be more aware of how close to the line you'd like them to stay.