I suspect your starting point was different than OP's. He is describing (I think) a crippling nervousness. This is very different from just tensing or run-of-the-mill nervousness.
All your advice is great, but (from personal experience) it's not enough to overcome phobia-level tension.
I think many people don't understand what phobia-level tension even looks/feels like. My own worst moment: presenting an architectural diagram to a VP, I started panicking, and repeated the phrase "...and we're going to build an abstraction layer..." three or four times in a row, in a slow monotone voice, until someone in the room snapped me out of it. Embarrassing is an understatement.
- Relax and meditate
- Practice a LOT. Everyday. Even two minutes helps.
- Learn to breath properly
- Record yourself speaking aloud at home with your phone. Make notice of all the nuances of the voice
- Prepare your material very well
- Visualize you are explaining the stuff to a friend at home. It will help you sound more natural.
Getting nervous is normal and part of the game.