When it comes to clip art, avoid clutter and the same, tired old art that everyone uses. Use art or graphics that illustrate a point or add to the presentation. Diagrams and tables must be simple and uncluttered in order to be readable. Cartoons can be inapropriate in a business presentation. Be cautious in using them Don't change type fonts.
Consistency is more elegant than excessive variety.
Avoid reverse type, light colors on a dark background are difficult to read. Use italics sparingly as they can be difficult to read. Delivery is where many presenters fail. Practice, practice, practice before you get in front of the class. Don't read the slides verbatim.