1. Little to no preparation
Even professional speakers practice relentlessly. Their skill is in relaying their message as if it is for the first time. Very few people can wing it and do an exceptional job. Don’t chance it. Practice, practice, practice.
2. Not knowing your audience beforehand
Good speakers always know who makes up the audience. It makes it possible to tailor the speech to the group and pepper the presentation with nuances only they would understand. Show that you actually care about whom you are talking to. (Yes, I know I ended the sentence with a preposition, which irks me, but whatever.)
3. Poor visual aids
Visual aids are meant to enhance your presentation. Period. They should not be overpowering, nor should they be minimal to the point of being negligible. Too much can be just as offensive as too little. Use them correctly, or don’t use them at all.
4. No eye contact
In conversation, we tend to be wary of people who don’t look us in the eye. This rule doesn’t change for a speaker in front of the audience. You may not be able to look at every single audience member, but glances should fall in various directions. If that makes you nervous, look between people. They won’t be able to tell.