Steal This Presentation
While this is a decent set of tips for <i>design</i>, the important thing to remember if you are actually giving a talk is that <i>you</i> are the presentation, <i>not</i> your slides.<p>Edit:  That said, the right visual aids can be important for delivering a powerful message.  I found this pretty useful in terms of making great looking slides that will actually help your talk by highlighting key points and keeping people's attention.