Keep it simple.
The second biggest mistake in a presentation is making your slides too busy or too complicated.
The best slide is often a single, simple picture with no text at all. If you put text on a slide, you are telling the audience to read instead of listen.
Once they miss some of what you were saying because they were reading instead of listening, you are no longer communicating effectively. Study how to make your slides understandable so your audience can invest their mental energy into listening, instead of figuring out your slides. Only include the bare minimum on a slide to back up what you will be saying during the presentation.
If you must include a complicated diagram, build that diagram over a sequence of slides that adds details a little bit at a time. If you feel inclined to create a busy slide, ask yourself why. Maybe you are emotionally attached to some less-important information and just “really really” want to include it. Maybe you are afraid to get a question about that less-important information. Get over this and delete that information from your slides. If you must, create an appendix for the less-important information and use the appendix only if a question arises.