For me the key factor was to allow myself to get into these tension-inducing situations in order to get used to them. From talking to authority figures to talking to groups of people to giving talks to a small audience to giving speeches to large audiences. Whenever I one-upped myself with a situation that seemed scary beforehand, every "smaller" situation that was scary before seems less stressfull in comparison the next time you face them, allowing you to relax and be more secure.