When I knew I had to talk, I was breathing in an certain way in order to focus on my breath and not in the anxiety of speaking. Inspire for 3sec, expire for 3sec, inspire for 3, expire for 4sec, inspire for 3, expire for 5, inspire for 3, expire for 4 ect.... This helped me in certain case to reduce my stutter at the beginning. Hope it will do also for you!