Keep your "visual representation" simple and standard.
If you're on Vista hide your desktop icons and use one of the default wallpapers. Keep your Visual Studio settings (especially toolbars) as standard and "out of the box" as possible. The more customizations you show in your environment the more likely people are going to focus on those rather than your content.
Keep the content on your slides as consisce as possible. Remember, you're speaking to (and in the best scenario, with) your audience so the slides should serve as discussion points. If you want to include more details, put them in the slide notes. This is especially good if you make the slide decks available afterwards.
If someone asks you a question and you don't know the answer, don't be afraid to say you don't know. It's always better than trying to guess at what you think the answer should be.
Also, if you are using Vista be sure to put it in "presentation mode". PowerPoint also has a similar mode, so be sure to use it as well - you have the slide show on one monitor (the projector) and a smaller view of the slide, plus notes and a timer on your laptop monitor.
