The hardest problem to overcome is called "data dumping." This is where we give too much information in too short a time. When we design presentations, we often think that to present well, we have to tell the audience EVERYTHING that we know about the topic all at once. In reality, though, a few points covered well will be better received than a bunch of points covered in a cursory fashion. We suggest that you limit your main points to five or fewer.
If you feel like you are making one or more of these mistakes when you design your presentations, make sure to view/listen to Podcast #3 How to Design a Presentation Quickly. We also have an Online Presentation Course that will walk you step-by-step through an easy process to create a great presentation.