Look at their eyes and read their expressions. Talk to them. Communication is two-way even if you're the only one who's talking. Think of your talk as a story, and be entertaining. Describe your work as you would tell an interesting story.
People like to be entertained even if it's serious research.
Give adequate background and motivation. Tell them what you're doing and why you're doing it. Motivate them to care about what you're doing. Relate what you're doing to something they might care about. State your main results carefully without lots of detail.