When you deliver a presentation, your audience members will be able to process only one source of information at any given moment. That is, they will either listen to your speech or read your slides, but they will not be able to do both at the same time.
Here is a short paragraph on attention, written by Dr. John Medina, a molecular biologist and the author of the book, Brain Rules:
"Multitasking, when it comes to paying attention, is a myth. The brain naturally focuses on concepts sequentially, one at a time. You can walk and talk at the same time. Your brain controls your heartbeat while you read a book. Surely this is multitasking. But I am talking about the brain’s ability to pay attention. It is the resource you forcibly deploy while trying to listen to a boring lecture at school. This attentional ability is not capable of multitasking."
If you are using a presentation software like PowerPoint, Keynote or similar, make sure that your audience can perceive and understand your slides almost instantly. This way, they will comprehend your visual message and still continue paying attention to what you are saying.
Treat your slides as if they are billboard signs next to a highway. Drivers usually have less than 3 seconds to read the contents of roadside billboards. How much attention would you give one with loads of written information on it? Similarly, you should have your audience be able to focus on you as the speaker, instead of investing too much time and cognitive resources on a slide with heavy text.
Here are 3 quick tips on how to achieve simplicity in your slide design:
1) Create breathing space.
Start with empty slides, instead of the software defaults. Presentation software usually greets you with a template that asks for a) title b) subtitle(s). This is almost a recipe for disaster. When you start with a completely empty slide, you will want to make sure that everything you put there will have a reason to take visual real-estate. Do you really need to put your logo on every slide? Can you do without the flying banners that came with the predesigned template? If whatever you put on the slide does not add any meaning, don't place them in the first place.
2) Bigger is better.
When placing text on slides, keep it as short and as large as possible. It should be quick to read. It should also be easily readable by people in the back of the audience. Use non-serif typefaces with a font size of 30 or more.
3) Simplify. And then simplify some more.
If you are working on an existing presentation, look at the slides and see if they contain anything that can be removed without losing its meaning. Also, consider dividing a heavy information slide into 2 or more separate slides. It will make it easier for the audience to digest small bites of visual data instead of large blocks of text.