What I found helping the most was actually doing a bit of reading aloud every day - like reading a story. I notice that I speak differently, and more carefully, and end up using my diapraghm naturally so I can have more control over my voice. When I read aloud a bit every day (like for my wife or daughter), I notice I feel some muscle memory kicking in, and it feels more natural to speak without trying in public situations with strangers where I might stutter. On tougher days if I stop reading aloud and get stuck, I end up sucking in giant amounts of air into my diaphgraghm, which is super inefficient and trying too hard. So in the end I do think it comes down to consistently building that muscle memory - less so the technique, and moreso something your body just remembers to do.