It is this: tell children exactly what you want, before you want it.
That's it. And yet it is surprising how rarely it happens.
Most behaviour problems in classrooms occur during transitions — when children move from one activity to another, enter a room, pack away, or wait. These are the moments when expectations are least clear, when the gap between what an adult wants and what a child does is widest.
If you want the class to come in and sit silently, say so — before they come in. Not while they are coming in. Not after they are already seated and talking.
The technique works for three reasons. First, it removes ambiguity. Children cannot follow an expectation they do not know about. Second, it removes the need to correct, which is always more difficult than preventing. Third, it treats children with respect — you are sharing the plan, not springing a test.
This is not about being scripted or robotic. It is about developing a habit of thinking one step ahead and narrating that step to your class. "In a moment, I'm going to ask you to…" — five words that prevent more behaviour problems than any sanction system ever could.
Think of a chess player who announces their move before they make it. The move is the same — but the relationship is different. You are a player who knows the game, not a referee waiting for someone to break the rules.
The simplest technique. Almost always the most neglected.