Keeping your audience's attention
Don't Be Boring!
Start your talk with energy and passion. Show excitement over your topic and try to convince your audience to share that excitement.
As your talk continues, realize that you'll have to work continuously to hold their attention. Plan in jokes, surprises, or other emotional events. Jump off the podium and stride out into the audience. Shout something. Admit an embarassing truth. Turn off the projector. Anything that breaks up the monotony and gets them to tune in again.
One great technique for holding interest is making your talk highly interactive. Interactive talks are more interesting than lectures. I start almost all my talks by saying "The point of this talk is not for me to reach the end of of it. The point is for me to teach you something. So if I lose you at any point, interrupt me immediately. I love questions." I tend to get a lot of questions, which is great. People usually comment positively on how interactive my session felt. Further, no one tunes out faster than someone who doesn't understand what you're talking about. Encouraging questions gives you the chance to not lose that person.
Remember: an audience's attention must be constantly maintained with energy and interesting content.