The only techniques that work for me are- Speaking slowly. Speak with even emphasis on all syllables and don't over emphasise on any syllable. Speech is basically sound+airflow. You might have noticed when you stutter your exhalation pauses. So try to not pause the exhalation process as much as you can. And take a big breath before you're about to speak a long sentence. And stretch the syllables that make you stutter while you speak. That will reduce blockages. And one thing that works for me but may not for you is that I change my accent while i speak a word that i know I'm gonna stutter at. All these techniques work for me 70-80% of the time. It's a lot I know but i have gotten used to it.