The first few minutes of agitation or frustration are crucial in determining how or if an incident is going to play out. Your response in those few minutes can prevent or fuel a potential incident.
Use anything positive that you can think of and praise, praise, praise. If historically he or she has hit or broken objects then praise them for not using their hands and refraining from violence. If they tell you why they are upset then tell them what a great job they did expressing their feelings. Use any positive feedback that you can think of.
Distract. I have seen a situation deescalated when a staff started singing the students favorite song. It was brilliant and changed the mood from angry to happy in seconds. Bring up good things that are coming up but bring them up as a reminder of good things to come not as a threat.
Ignore. Be cautious with this technique. There are kids who will stop once they see that they are not getting attention but others may raise the stakes until you attend to them. Try this but pay close attention to their response and be prepared to act accordingly.
Stay calm. Use soft voice tones and calm facial expressions regardless of how escalated your child gets.
Most importantly, allow your child a face saving way out. Avoid an audience when possible and once the situation is over- it's over. Reliving the episode will only add anxiety and can lead to further unwanted behaviors.