This should go without saying, but it's imperative that your PowerPoint presentation be free and clear of grammatical and spelling errors before you give it. You might make mistakes or stutter while speaking, especially if your presentation is long or you don't frequently give them, but having a picture-perfect visual impression is one of the key steps in impressing your class. This is particularly important if you plan to talk over each slide for longer than a minute or so -- in other words, long enough for audience attention to drift to the slide itself -- or if you plan to give classmates a print-out.
Although PowerPoint affords you the option of enhancing your presentation with graphics, videos and sounds, you should use these elements sparingly, as they can (and, in most cases, will) distract your audience from whatever point you're trying to make. The same goes for flashy fonts and backgrounds. If you're giving a presentation about different types of flowers, having pictures as visual aids is appropriate. If you're talking through the history of the Crayola crayon company and want to have each slide represent a different color in the box, however, you may just give your audience a headache. When in doubt, keep it simple.
A PowerPoint slide is not designed to be like the "teleprompter" machine from which politicians and newscasters read. You shouldn't place everything you plan to say on the slide; you should simply highlight your key points or main ideas so that even casual or disinterested fellow students will be able to take away the gist of your discussion. As is the case with limiting your use of multimedia, having more streamlined text will help keep your class' attention focused.