Try a chart, graph or illustration to clarify what you're talking about. Use pictures, graphs, charts, and color for variety and to draw the audience's eye to your most important points. Be sure the slide remains uncluttered. This gives you freedom to move about and permits you to travel among and interact with your audience. Keep the lights in the room as high as possible without washing out the image on the screen.
Dark rooms lead to inattention, occasional sleeping, and they make it impossible to take notes.
Besides, you can't be seen in the dark. Sometimes it is possible to remove the light bulbs or loosen tubes that shine on the projection screen. Have your presentation in a variety of formats, especially if you're giving a presentation in a distant location. Consider print, overheads, 35mm slides, as well as computer slides. That way, no matter what happens to the equipment, or if the attendance is larger or smaller than expected, you can adjust to the circumstances.