Okay, my 7 year old was a chronic hitter. For like, years. We tried e..v..e..r..y..t..h..i..n..g. For the last, oh, 3? months she has *not* been a chronic hitter. In fact, for the first time ever it's *rare* that she hits (and hopefully that will continue-that it's rare, I mean). I can tell you what has helped us.
Recognizing sensory issues: Dd has a lot of sensitivities-clothes feel funny, socks feel funny, super-hearing, that kind of thing. We never connected any of her hitting with her sensory stuff until her psychologist said she suspects that dd's siblings' loud noises and close proximity cause dd to become agitated-activate the old fight or flight response- and she lashes out, gets too upset to think first and find other ways of handling the problem. Recognizing sensory triggers allows us to help dd avoid or cope with them, so that she can behave better. Example: bought her hearing-protection headphones to dampen noise, dd begins to get agitated, we remind dd to put on headphones, dd puts them on and voila-complete mood change, problems averted.
Looking for triggers: It really, really helped to carefully observe under what conditions she was hitting. Who was doing what? What was happening? What time was it? When did she last eat? What had she been eating? How had she been sleeping? What had her mood been like? And so on. Once we saw the triggers, we could be a lot more proactive-helping her deal with things before she reached the point of hitting.
Reading The Explosive Child, attending workshops on parenting the explosive child, and using the Collaborative Problem Solving approach from this book: this has helped us both reduce the amount of hitting (and raging tantrums) and teach our dd the skills she needs to handle frustration without hitting (or raging tantrums). The authors say that in order to use their approach, the child you're working with needs the skills of a typically developing 3 year old. I use it with my 3 year old and she can do it.
Reading Raising A Thinking Child: lots of ideas to help kids learn better communication, perspective-taking and problem-solving skills.
Empathy and connection, first last and always: This is part of the CPS method too. I was always saying: "No hitting! I can see you're angry..." This is instruction/reprimand first, then empathy. I also did a lot of "empathy, but": "I know you're angy, but you can't hit." I have learned that when I do these things, dd does not feel heard and dd does not hear me: the result is that dd does not learn. She's very intense, emotionally. She needs to be heard first, to know and trust that I am working *with* her. So she needs empathy first: "(dd hits/tries to hit) whoa! what's up? (listen, or guess) So, you wanted that toy, and he has it. (wait for agreement or correction, do this until I'm sure I have heard her concern correctly.) I understand. Thing is, I need us to be safe, hitting isn't safe, and brother doesn't want to be hit either. Let's see if we can work this out another way." You get the idea. It's even better if I don't say anything about the hitting right then (and really, she knows I don't want her to hit) and just focus on the problem at hand: "you want that toy. Thing is, he isn't done using it. Let's see if we can work it out together with our words. Do you have any ideas?" The empathy keeps her calm, and once she has been heard she is open to hearing me-and I'm modeling for her other ways of communicating and handling frustration. KWIM?
Also, we are vigilant about her getting enough sleep and good nutrition. Food dyes are bad. Not getting enough good sleep is bad, leads to worse behavior. And we are big on helping her learn to relax, and to recognize when she's getting upset, and to calm down once upset.
We totally focus on proactive measures. This is hugely important. Dd is not learning in the moment she is hitting, this is not the best time to teach about hitting and using words. We have to work on proactive problem-solving, and on jumping in at the first sign of trouble to guide her (not so much anymore, thankfully, she's learning).
On the occasions that dd has kept hitting and hitting, I've taken her to a safe room and closed myself in there with her (she won't stay by herself, just escalates things) and quietly wait until she's calm. On a couple of occasions I have restrained her-not to stop the hitting, but to keep myself, her or her siblings safe.
There's probably more. We've worked on so much. HTH. (eta dd is also in therapy with a psychologist for anxiety, which is another contributor to her behavior.)