[I am republishing this page from Seven Deadly Mistakes Presenters Make, because the original had an error on it; unfortunately Typepad is not letting me change the original post right now.]
Mistake #3: Only including evidence that supports your recommendation.
It is tempting to include only facts and arguments that support your case in your presentation, because you want to strengthen your case, not weaken it. However, all the empirical research confirms that audiences will find you more credible—and more convincing—if you also include the arguments against your recommendation, and then carefully rebut each one of them. Lawyers call this “stealing thunder”: if you bring up an objection first, that objection has far less force than if someone in your audience does.
Mistake #4: Using color, sound, and clipart to make your presentation look professional.
Adding all the embellishment that PowerPoint allows you to may make you feel more professional, but it harms your communication. The research is unambiguous here also: any added color, sound, or image that does not directly reinforce the specific message on your slide will distract your audience from that message. Animated slide transitions, in particular, are almost universally destructive.