5. Be in role
Don't, actually, be yourself. If you are yourself, then you will feel more threatened, and therefore more nervous, because it will be Rakesh Patel or Isabella Jones who might mess up, and everyone will remember. If, however, you are not Rakesh or Isabella, but the opening speaker for the opposition (Mace) or the Deputy Prime Minister (British Parliamentary), then it will not be you who is on show, but a character in the drama that is the debate. The convention in BP of speakers addressing each other with the title of their role will help you to detach yourself even further from your everyday self.
Speaking in role will also make you a more effective speaker. You might have been given the task of proposing the motion in favour of making all school lunches vegetarian, when in real life you are a massive steak lover, or passionately believe everyone should be able to eat anything they want. In that case, it could be hard to speak convincingly. If, on the other hand, you take on, like an actor, the role of someone who believes with every fibre of her being that the very notion of eating dead animals is cruel, unnatural, and a disaster for the environment, you will be far more persuasive.
One student of mine suffered terribly from Obsessive Compulsive Disorder. She was plagued constantly with disturbing, distressing thoughts. She volunteered for debating to help her to overcome her anxiety. Not only did she do debating, she did it superbly, going on to be on the winning team in the sixth form debating competition, speaking in the final in front of the whole of Year 12 and 13. For the five minutes she was speaking, her anxieties were forgotten, because she was speaking not as herself, but in the character of a speaker for her side of the motion. Debating helped her to be less anxious not only about public speaking, but about life itself.