One of my managers just gave me a wonderful tip that I thought I would share here to help the community.
What makes impromptu speaking difficult is the high expectations we attach to it. It gives us performance anxiety!
We can lessen the stress by reminding ourselves that there is no "right" way to talk. Sometimes it helps to aim for just getting the job done.
So don't focus on making an impression, focus on getting the job done!
Hope this helps!