Keep a pattern. It's pleasing to your audience and allows them to quickly sort out the information. Use familiar layouts. Stick with the same fonts throughout your presentation. If you use an image effect when placing a picture, use it on all your images.
Don't use complete sentences. Imagine the words as a backup singer. Your audience should not have to stop listening to you in order to read.
The larger your audience, the larger your font size should be.
Stay with your audience. A glance at the slides is fine.
Classic sign of an over-enthusiastic student is a document covered with borders, text art and image effects. The very best presentations are remembered for their substance so don't let the styling get out of control.