One strategy that I have found to be effective is reviewing the classroom rules and expectations. The children are able to recall the rules and the consequences for negative behaviours. They can also explain why the rules are necessary and why they should be followed. What eventually happens is that the children themselves will notice an misbehaviours among their peers and they will remind their peers of what should be done. It is quite interesting to observe this in reality, preschoolers take this matter very seriously (lol).