Contrast - choose suitable colour (colour wheel), image (background) and fonr (serif & san serif)
Alignment - text and picture put in parallel or in balance or symmetrical
Simplicity - present your presentation simply with easy understanding and use text not more than 3 type of font with effect that is suitable according to occasion
Proximity - organise text and graphic with suitable space
Emphasis - must have one main item to focus on
Repitition - use consistent colour, font and graphic
Read more at http://www.slideshare.net/shimamohd/6308-casper-presentationupdated2?related=1
other than that, i do some search on web 2.0 education for google presentation.