The first thing that you have to keep in mind is that a prop is a supplement to your speech. It is your speech and your visible presence that are more important. This is not to say that I wish to undermine the importance of props because it is also true that props can greatly enhance your performance if they are used properly to create the right effect.
You can be sure that props help to create deep impressions in the mind of the audience because they appeal visually. It is common knowledge that we remember images better than words.
It may sound trivial at first, but it is important to consider where you are going to stand with respect to the flip chart or the projection screen. If you are right handed, the flip chart or the screen should ideally be placed on your left. This would enable you to write on the flip chart or use your pointer on the screen without blocking them from the audience. The placing is reversed for left-handers. It is also good sense to place these props at an angle.
While using an overhead, make sure that you can place the transparencies right side up. We do not use overheads everyday, and so you have to be conscious about what you are doing with the transparencies! You should also take care to turn off the overhead when you have finished for the time being. Keeping it turned on will leave a splash of white on the screen as well as initially make the audience falsely expect something to come up! A little bit of practice will make you avoid such situations and make you confident in your handling of the overhead. Regarding the information or the images in the transparencies, they should ideally be kept simple. Complex information or images will surely confuse the audience and gradually make them indifferent to your presentation.
This last piece of suggestion also holds good for PowerPoint presentations. What holds good for both PowerPoint and overhead presentations is that they should not be dominating. What I mean to say is that they should be of secondary importance with respect to your speech. It is good to remember that if the need arises, you can sort of black out a PowerPoint presentation by pressing the B key, and resume it by pressing any key.
You may or may not have to talk during the presentation. That will depend on what you are presenting. Written information need not be read out. If you read them out, you will be interfering with the reading process of the audience. Keeping the information clear, precise and legible is all that you have to do. Use colors to bring variety. However, never overdo. Keep to simple basic colors. These do not betray you even if you are not an expert in understanding the combination of colors.
But when you are showing diagrams, it is always good to make a simple commentary even if the diagrams are simple in nature. This helps in two ways. Firstly, your explanations help the audience to relate one image with the other and follow the transitions meaningfully. Do not take it for granted that every member of the audience have the same power to grasp visual images however simple they may seem to you or others. Secondly, and more importantly, this talking, however little, makes your presence always felt.
You can use a video show as a prop. Almost anything can be used as a prop. The microphone you are using is a prop. So is the lectern.
Never attempt to use any special props if you are a beginner. These may spoil the whole show if you are not able to use them with the right effect. Veteran speakers use such things very successfully. When you graduate to being one, you can create special props for yourself. You shall then be able to transform everyday things, toys, and what not into meaningful props.
Till then, happy sailing!