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Do not read directly off a screen. If you are going to use Power point or another slide show software, use your slides as an introduction, put brief points on the slides and explain them more in depth while speaking. If you can also do the presentation from memory having direct eye contact with your audience with out using flashcards or reading off a screen, the presentation will come off more professional then if you keep looking away, appearing to not know what you have to present. One last thought that will help is to practice your presentation before arriving at the demonstration and time it, to make sure you can come in a time limit if ones given at the demo.
Good luck and keep us updated.