Choose a simple PowerPoint template and apply it consistently across your deck. Honestly, I'm not even sure how people manage to do this, but I routinely see presentations with a ransom note assortment of fonts, sizes, and styles on each slide. PowerPoint goes out of its way to make it easy to apply a consistent style; take advantage of it. In PowerPoint 2007, for example, click the Design tab and choose one of the themes. And keep it simple; make sure the text is readable against the background, and that there isn't a busy design competing with the text. Remember what Halle Berry told me once in a dream: Just because a theme exists, doesn't mean you should use it.
Less is more. Don't feel the need to pour your entire speech into your slides. As a rule of thumb, each slide should list 3-5 main points in bullet form. You're in the room to expand on those nuggets verbally. The slides are there to enhance your overall presentation, not to replace you. If it takes more than a few seconds to process the text on any given slide, it's too much: Your audience will spend its time reading instead of paying attention to you, and probably end up doing both of those things somewhat poorly.
Test your links. If you embed video or links to content on the Internet or a network share, for the love of Woden test it before you walk into a conference room and try presenting it to a dozen people. I can't tell you how frequently I encounter folks that don't know how to launch an external link from within their deck or who have linked to an obsolete version of the document they wanted to show. Do you homework.
Test your projection. Along the lines of testing the innards of your presentation, be sure you know how your laptop connects to the projector. As a rule of thumb, make sure the laptop is fully booted and then connect the VGA cable. If you're on Vista, you should then choose Win+X to open the Windows Mobility Center and click Connect Display. (If you're testing Windows 7, it's even easier: Press Win+P to open the projection settings directly.)
Finally, if you're sharing your deck with attendees, be sure to check out my recent tips on keeping your PowerPoint deck small enough for e-mail.