These items are key, in many respects, to surviving a speech in front of an audience:
1) Knowing, and
2) A concern for the listener that approaches being a missionary.
Knowing: you will realize that you know beyond doubt when your boss asks you to fill in for him/her. That should give you the perception check you need to accept your knowledge base as being dependable. First, your boss isn't going to select anyone to be their representative anywhere unless there is confidence in their ability. Build on that confidence. When you are chosen to speak it is because there is confidence in your ability. Accept that and build on it.
Missionary: think of your goal as being more important than your ability to avoid mistakes and shine under stress. Instead, think of your knowledge as being something that individuals in the audience need in order to realize their goals. The important thing then is that they hear, be able to recall and use what you are presenting to them. Nothing else should be allowed to supersede that goal.
These two concepts should go a long way to reduce the fear or stage fright that is no natural to most of us.