This is a useful tip I've handed out since it was given to me years ago.
At the time, in my 20s, I was excitable, jabbered on and on when talking to people, and was concerned -- given my job at the time -- about the impression I made on people.
A mentor advised me to lower my voice when I talked. When we're nervous, our heart rate goes up, we breathe faster and more shallowly, we talk faster, and as we talk faster, our voices rise in pitch.
So when I caught myself talking fast or feeling nervous as I talked, I reminded myself to lower my voice. The change in attitude was almost instant.
As my pitch fell, I could feel my voice's center move from my head down to my chest. Lowering my voice slowed my speech, which slowed my breathing. I could feel my shoulders relax (I hold a lot of tension in my shoulders).
And I could feel my mind slow down too. My thinking evened out, I took my time, and I usually felt like I was now consciously participating in the conversation, not flailing like a fire hose.