Not thinking about what you're going to say is doable, but you're risking messing up in other ways. What helps me is to focus on the NEXT word, or the next sentence. That keeps your mind off of what is coming out of your mouth and onto what comes next. It takes the pressure off of what is currently being said. Also I know it's the most common and most difficult-to-follow advice, but try and stay calm. Let your throat relax and keep your heart rate as normal as you can with some steady breathing.