It's really difficult and I don't think there's a one-size-fits-all approach.
When we were going through the various assessment processes for DS, I was interested to see that in all the questionairres I was asked some version of, "what are his strengths?" or "what do you love about him" and I think that was actually a very useful exercise. it reminded me of his strengths and good points, and I realised after it also gave me a starting point for positive reinforcement with him - working these things in to conversation or paying attention and praising him for them etc. Really importantbecause he was getting so much negative feedback all the time.
If there are things he wants to do/ friends he wants, are there more healthy ways you can support this? Eg joining up to a club that is "cool" or even just buying that one ridiculous branded item. I'm not talking about going crazy, but showing some support, but in a way that's not destructive and unpleasant?
If he has things in his life that ARE good and where he has previous good relationships with adults in those, can you encourage more of those and/or speak to those adults and ask them to intervene? We got DS' sports coach to have a word with him - he was getting really negative and iffy about his sport, not wanting to turn up etc. A lot of it came down to lack of self confidence and a chat with his coach really helped.
Let go of as many of the smaller things as you can. That doesn't mean putting up wth really shittybehaviour, but I think that sometimes you do have to let some minor elements go.
What incentives are in place for positives? We've had short quick incentives for, for example, not getting codes at school for a week, with bigger incentives for more long term positive behaviours.