This relieves tension in the room. Don't organize your talk in advance. It is usually best not even to think about it until your name has been announced by the session chair. Above all, don't write the talk out, for it may fall into enemy hands. Never, ever, rehearse, even briefly.
Talks are best when they are given spontaneously with thoughts organized in a random fashion.
Leave it as an exercise for the listener to assemble your thoughts properly and make some sense out of what you say. Discuss each slide and overhead in complete detail, especially those parts irrelevant to the main points of your talk. If you suspect that there is anyone in the audience who is not asleep, return to a previous slide and discuss it again. Face the projection screen, mumble, and talk as fast as possible, especially while making important points. Wave the light pointer around the room, or at least move the beam rapidly about the slide image in small circles.