I found for ds that the key was definitely to reduce his stress levels overall but that wasn't easy without the school's cooperation. I think those with supportive schools will certainly have more success with anger management than those who don't.
As many of you have found, there is little point in trying to address anger when it's in progress. Prevention is the best way forward so you'd need to familiarise yourself with your child's triggers, as well as what they respond to best for calming down. They may not realise these things for themselves and have to be taught to recognise them, and act upon them.
I used a multi-layered approach for ds with lots of linked strategies that had an overall aim of teaching him to recognise degrees of anger and to take responsibility for the consequences of losing control. I used social stories which helped to explain appropriate behaviour and responses to difficult situations, and took advantage of 'in the moment' opportunities to model good stress management. We had one that looked at what would happen in meltdown and emphasised his responsibility for bringing himself out of it, and for any behaviour that happened whilst he was angry. All of these were discussed at quite and calm times so that I could be sure he absorbed the strategies without any anger to distract him.
The Incredible 5-Point Scale was a very useful tool to demonstrate the degrees of control (or lack thereof) from calm to meltdown. We had several, all with different themes depending on ds's interest at the time - Doctor Who, Star Wars, Bionicle, Lord of the Rings... Again we'd go through these at calm times to embed the best approaches and help him recognise where he was on the scale and how to avoid escalation, and hopefully to bring himself down as well.
A key strategy to engage ds was a set of six monthly SMART targets with a significant prize at the end if he achieved them. Early days it was a big Lego set, and the last one we did got him his first mobile phone - it had to be something significant and important to him so that he'd be on board. We'd set four or five key targets for him to aim towards and each week we'd talk about his progress and what we could BOTH do to help him achieve them. This empowered him to recognise that anger control was HIS responsibility but he also knew that I was there to help in any way he needed. There was also a visual line of progress across the room where we talked so he could see how far he was towards his goal, and I think that helped him to understand the aim better.
If he actually did get to meltdown (and oh boy, he did SO often to begin with), we had a 'contract' for what would happen when he lost control completely. We had an agreement that I'd leave him alone in his room (he always retreated to his room and slammed the door behind him) and return after fifteen minutes to ask if he wanted my help, then again after another 15 minutes if he didn't. This gave him time to calm down but it also removed any confusion about expectations if he got into this state, which I'd learned from experience would escalate his frustration and add to the rage. I had printed cards to give to him each time it happened so he knew that this was 'meltdown' and the agreement would kick in.
I also thought it important to engineer lots of achievements because I believe that self-esteem is a key feature in anger. I made him certificates for positive behaviour and we kept them in a file to look at over and over, to remind him how well he was doing. Lots of positive social stories too (it's often forgotten that 50% of social stories should reflect achievement, and not just behaviour modification).
All of the tools I used were created on the computer and I have them all on file, if anyone would like to see them. You'd be welcome to use them as templates if you think they'd be useful for your child. Then again, not all strategies work for all children, and the key is to make everything you do personal to the individual.
There is no quick fix to anger management but it can be done. Ds is 15 now and hasn't lost his temper to a significant degree in about three years. Hard work but worth it .