What generally makes m perform better in presentations and generally when speaking - is knowing that I have the balls to do it and so I've always done it. That is to say - throw caution to the wind, if you want to do you're presentation do it. What helps me alot is performing it out in front of a small group of people, it's less intense and allows you to pick up on any blocks that occur during the performance, from there I substitute the words that make me block for easier ones. Then I practice my 'performance' voice, it's the voice I use when I HAVE to speak at length (eg. telling friends a story) my voice goes deeper, I speak slowly and with emphasis - and should a block come up I'll close my eyes, focus on breathing then use my hands theatrically to ease the word out (or substitute the word) - it is all very dramatic but it helps to build a powerful presence which is calming and reassuring. Before presentations and interviews (had one today, went ok) I also recommend listening to music that puts you in the mood for the event (my favourite is banging Hip Hop) then after that I mentally shut out everything and solely focus on the task ahead of me - eye of the tiger baby!