The file Behavioural_Strategies.pdf is highly recommended :)

Avoid using 'demand' words such as:
 

1. Need
    
2. Must/Must not
    
3. Will/Won't
    
4. Can't
    
5. Now
    
6. By (a time/date)

1. Is it OK with you if---
2. How do you feel about---
3. Do you mind (doing/going etc.)---
4. Would/could you---
5. If you're happy to---
6. When you have finished with---, could you then---

'The Golden Rule'

Demonstrate empathy - overall, and as a primary means of de-escalating behaviour. Always demonstrate empathy towards the child first, before taking any other action such as trying to explain your point of view to the child. Most parents are too quick to explain a situation to the child, to provide guidance or teach a lesson. But a child in a meltdown state will not calm down until he or she feels like they have been heard and understood. Empathy is critical not only to de-escalate a crisis, but for your child's overall mental well being - They need to be validated.
If you are unsure how to show empathy, techniques include mirroring the child, asking what happened, listen and then validate by repeating back their words ("wow, I can see why that was so upsetting to you, John took your FAVOURITE pen! That would make me mad too"). SO, demonstrating empathy first, before trying to express your point of view, even if you don't agree with your child, is a proven and effective strategy to make your child feel validated and understood, and will help calm them down in order for you to provide a teachable moment.
You can be flexible and calm, but none of that matters unless the child feels understood or "heard".
By the way, this technique works even on NT adults too!!! The minute you empathise, the other (upset) person will begin to calm right down.

Of course, sometimes that empathy is not easy to demonstrate - sometimes you as a parent are frustrated. It can take a lot of effort, but it is worth it!

Thank you to Christine F for this very perceptive strategy :)

Sometimes great success can happen by making 'things' into a game - like getting dressed, brushing teeth, having a shower, getting out to car, etc, etc... but be aware that the interest can wane suddenly, so be careful not to over-play the activity, and certainly avoid any and all hint at manipulation - and ALWAYS have a Plan B, C, D, E, etc... ready in the wings :) Keeping it fun can achieve a lot :)
Failures happen in life, even with NT kids, so try not to be downhearted if it does not work first time, or every time :)