A good PowerPoint presentation will convey your message, please your audience, and make you look smart. You can refer to this PowerPoint example for tips on how to make a simple, clear presentation. Then you might want to take an amusing look at common PowerPoint mistakes:
When you boil it down: tell your audience what you are going to say, say it, then remind them what you said. Keep your audience in mind (are you talking to undergraduates? faculty and graduate students? meteorologists?) and try to get the level of the talk correct, even tailor the talk to that group. Never make a statement that you can't back up. Finally, don't forget to answer the questions "Why is this important?" and "How does this relate to the bigger picture?" - once at the beginning of your talk and again at the end.
I print out a copy of my PowerPoint presentation and write what I plan to say for each slide on a sticky note attached to each slide; that way, I can practice any time (on the plane, waiting at the doctor's office, etc.).
It is very important to practice your talk in front of an audience of friends - you'll get good feedback and figure out if you need to add/remove/combine/change the order of slides, how long it will actually take you to give your talk, and discover whether there are gaping holes in your talk's content (more introductory material, more thorough explanations of key points, whether adding a figure would help you explain a point, etc.). Hopefully, you will also feel more confident about your talk after practicing.
When you're actually ready to present your talk, keep these things in mind. I particularly like three of those points: 1) Tell a story with your talk; 2) Use (relevant) photos/images/figures; and 3) Never apologize. Use bullets on your slides to remind yourself of key points: you can glance at your slide to remember what you'd like to say, and then make eye contact with your audience while you're speaking. And remember that when it's time for questions at the end of your talk, it's OK to say "I don't know".
