There was a time when presenters used 35mm slide projectors to add visuals to their words. Adding text to those slides required expensive equipment and took extra time. So presenters often displayed images without any text at all. The only slide design tip then was to make the images relevant.
This all changed when presentation software like PowerPoint entered the market.
Adding text became easy and many presenters began (ab)using this feature. Slide design took the back seat while content became the driving force. And with that trend, cluttered slides entered the scene.
You have most likely seen presenters showing slides full of bullet points. Often these bullet points include every word the presenter says. Like this example:

The problem with slide design like this is that people cannot read and listen at the same time. As soon as a slide appears, people reflexively look at it. When they see text, they start reading. And when they read, they listen to their inner voice and not yours.
5 Slide Design Tips to Create Attractive Visual Aids
1. One slide per message
Avoid putting two or more ideas on a single slide and then walking the audience through it. Your slides are a backdrop to add one more dimension to your message, not an infographic. Whenever you feel the urge to use a laser pointer, take a step back. Think about how you could represent that piece of information on a separate slide.
2. Edge to edge images
There is no need to frame the images and text on your slides. The screen’s edge already serves that purpose. Extending your images all the way to the edge of the available display space creates extra impact.
3. Large font size
When you add text to your slides, make the text as large as possible and use a contrasting color. Use all-uppercase letters only for titles or acronyms. Also avoid vertical text as that is hard to read for many people.
4. Stay consistent
Pay extra attention to the consistency from one slide to the next. Use consistent fonts, colors, and layouts. Choose smooth, uniform transitions between slides. Also avoid transition effects and animations that don’t serve a purpose.
5. Bring numbers to life
Represent numbers visually by choosing an appropriate chart style. Use Line Charts to show progress or quantity over time with the dates at the bottom and the values on the side. Bar Charts are ideal to display a snapshot of how different values relate. And Pie Charts show numbers that add up to one hundred percent.
You don’t have to be a graphic designer to make good looking slides. These slide design tips will already help make more attractive visuals. Keep in mind that slides are a backdrop and not the centerpiece of your presentation.
A final tip: review your slides as thumbnails to see which ones add signal and which ones add noise.
To learn more about creating slides that really stand out, consult Presentation Zen by Garr Reynolds and Slide:ology by Nancy Duarte. And for a humorous take on this issue, watch Don McMillan tell the world how to NOT use PowerPoint