I just read presentationzen by Garr Reynolds. It is a quick read with terrific graphics. I think the book is mostly targeted to business people who have no graphic or instructional design background, but it has valuable information for people who design training materials every day.
One of the concepts carried throughout the book is simplicity. Here's a great thought on how simplicity relates to appearance/design. There is a difference between design and decoration. Decoration is noticeable, like icing on a cake. Design is unnoticed. Design starts at the beginning; it is not an afterthought.
Another observation drives home the point about how important it is to start out strong. The author references Seth Godin, "Often, people come to a conclusion about your presentation by the time you're on the second slide. After that, it's often too late for your bullet points to do you much good." Wow, do you really want your second slide to be a list of learning objectives? I don't.
Here's some more great advice. Do only what is necessary to convey what is essential. Clutter, bulk and erudition confuse perception and stiffle comprehension. Simplicity allows clear and direct attention. Clarity and simplicity are often the only things people want or need.
It would be easy to go on, but if these topics interest you, your time would be better spent reading the book. This is a good one.