It's probably easier to list all the things you shouldn't do in Powerpoint than all the things you should. The following presentation by Don McMillan illustrates this:
As an illustration of how Powerpoint will never be a replacement for truly inspirational speaking, check out the Gettysburg Powerpoint Presentation.
The psychologist Stephen_Kosslyn, an expert on mental imagery, has written a guide to creating compelling Powerpoint presentations based on psychological principles. He lists three basic goals for a presentation and specifies principles (to be treated as guidelines, rather than rules) that underpin effective communication (see Kosslyn, 2007, p.18):
Goal 1: Connect with your audience
Principle 1: The Principle of Relevance
Communication is most effective when neither too much nor too little information is presented.
Principle 2: The Principle of Appropriate Knowledge
Communication requires prior knowledge of relevant concepts, jargon, and symbols
Goal 2: Direct and hold attention
Principle 3: The Principle of Salience
Attention is drawn to large perceptible differences
Principle 4: The Principle of Discriminability
Two properties must differ by a large enough proportion or they will not be distinguished.
Principle 5: The Principle of Perceptual Organization
People automatically group elements into units, which they then attend to and remember.
Goal 3: Promote understanding and memory
Principle 6: The Principle of Compatibility
A message is easiest to understand if its form is compatible with its meaning.
Principle 7: The Principle of Informative Changes
People expect changes in properties to carry information.
Principle 8: The Principle of Capacity Limitations
People have a limited capacity to retain and to process information, and so will not understand a message if too much information must be retained or processed.
The concrete applications of these goals and principles are discussed in Kosslyn's book. To give just a couple of examples of how (e.g.) the Principle of Salience might apply, Kosslyn suggests the following: (1) Build up a slide one part at a time, so that attention is focused on the current part; (2) If you are going to verbally expand on your slide, then move around the stage so that you keep the audience's attention - but stop moving whenever you switch to a new slide, video, or audio file.
Multimedia principle: Presentations using words and pictures lead to deeper learning than presentations using words alone.
Contiguity principle: Presenting words and pictures simultaneously leads to deeper learning than presenting them successively.
Coherence principle: People learn more deeply from multimedia presentations in which extraneous words, sounds, and video are excluded rather than included.
Modality principle: People learn more deeply from pictures and narration than from pictures and on-screen text.
Redundancy principle: Deeper learning occurs when words are presented as narration rather than as both narration and on-screen text.
Personalization principle: Deeper learning occurs when words are presented in conversational style rather than in formal style.
Interactivity principle: Deeper learning occurs when learners are able to control the rate of multimedia presentation than when they are not.
Signaling principle: Signaling key steps in the narration leads to deeper learning.
There are some commonalities between Kosslyn's and Mayer's princples. For example, the Interactivity Principle (Mayer) relates to the Principle of Capacity Limitations (Kosslyn).
Research into the effectiveness of Powerpoint for improving learning has produced rather mixed results, as summarised and discussed by Susskind (2005) and Savoy et al (2009). Generally, it seems that Powerpoint has a positive effect on student attitude and self-efficacy, but no consistent benefit on performance. However, the study reported by Savoy et al found that a Powerpoint presentation had different effects depending on the nature of the material to be recalled. They reported that students remembered 15% more verbally-presented information from a traditional lecture (involving a chalkboard) than from a Powerpoint lecture. Recall of graphics and alphanumeric figures did not differ between the two types of presentation. Finally, the authors looked at recall of audio-visual information - verbal information that was supported with the use of a graphic or alphanumeric visuals. This also did not differ between the traditional and the Powerpoint presentation.
Within each delivery style, Savoy et al (p.864) found "that in PowerPoint presentations graphic scores were higher than audio scores and in traditional presentations audio scores were higher than graphic scores, predictably." They argued that these differences within each style have the effect of suppressing the overall effect of delivery syle. However, it is not possible to determine from this and other studies to what extent the principles described by Kosslyn and Mayer were followed.