As a professional speaker who delivers over sixty talks per year, I know firsthand how technical malfunctions can leave even the most seasoned speakers feeling flustered and frustrated. When a slide presentation goes off the rails because of a power failure, audio glitch or other technical issue beyond the speaker's control, the best way to handle it is with some humor and a heaping dose of composure -- not to mention, a whole lot of preparation.

Murphy's Law states that anything that can go wrong will go wrong. The most in-demand professional speakers know that Murphy was an eternal optimist. And they know that preparation trumps all else when delivering a keynote presentation, especially if using slides.

I use slides in about half of my keynotes. Yet, unlike some speakers who rail against them, I don't mind the use of slides -- as long as they are used properly and in service of the audience.

That said, here are six common mistakes speakers make when using slides that you should avoid when giving your presentations.

Tiny Print

One of the questions I get asked often is: what size font should I use? My answer to that is: if you're asking what size font to use, it's probably too small.

Font size will depend, in large part, on the size of the screen, how many people are in the audience and how the room is laid out. The person sitting at the furthest point from the screen should be able to read what is on your slide. If you catch yourself saying to an audience, "The people in the back might not be able to read this," that's a clear sign the font is too small. The slides should complement the words you speak, not distract from them.

Too Many Words

If you have words on your slides, ask, "What do I plan on saying that isn't on the slide?" The easiest way to avoid reading your slides is to not put words on them.  I typically use no more than four words on my slides when I want to reinforce a concept. For instance, when I train sales teams on the concept of Same Side Quadrants -- Issue, Impact, Results, Others -- I show a slide to illustrate the quadrants with those four words. One picture with four words, that's it. I use it to provide a visual anchor that people can use to remember the concept.

Your slides should be easy to see and understand. If you're trying to pack too many words on the screen, it becomes too hard to read and harder to understand. Remember: less is more.

Duplicate Words

There's psychology that says humans have different learning styles; some people learn by seeing, others by hearing, and others by doing. Yet, there's also science that shows people are unable to multitask - at least not effectively. When it comes to slide presentations, if you use a slide with text on it, the audience can either read what's on the screen or listen to you speak, but not both at the same time. Here again, a slide should complement, not duplicate, the words you speak.

So when you bring up a slide with text on it and you want the audience to read it, you need to pause and wait twice as long as you think it will take someone to read. Then, once the audience's attention is back on you, you can resume.

Unauthorized images

Just because you can do a Google search to find an image, doesn't mean you have the right to use that image. Every image I use is either commissioned for a specific purpose or it's licensed. Rule of thumb: when in doubt, license it.

Unnecessary slides

It's very rare for someone to run out of slides during a presentation. In fact, it's quite common for people to rush through their slides because they have more slides than they actually need. What you need to ask yourself on every slide you create is, "What does this slide add that the audience would be missing if it wasn't there?" If you can't answer that question, hit delete.

The number of slides you use will depend on the the type of presentation you give and the impact you want to make.You can give an hour-long presentation with six slides, or a 20-minute presentation with 30 slides. Both can be effective if done right if you can answer the question, "What does this slide add that the audience would be missing if it wasn't there?"

Lack of preparation

If the purpose of your slides is to remind you what to say, you're using them for the wrong reason. If you really need to see your slides to make your presentation, then you're not properly prepared. Remember Murphy? Whatever can go wrong will go wrong.

I've given keynotes at an audio-visual conference where the screen didn't work; at an event where an audience member inadvertently kicked the plug on my projector; and at places where the power went out. On those occasions, I was able to move right along because I knew my material inside and out. I was able to deliver without needing any visual aids.

When you're prepared, you can navigate almost anything. And if all goes well, then slides are just a bonus.