Kathy Sierra over at "Creating Passionate Users" has written an incredibly important summary of the things that turn off your audience in the first ten minutes. She also gives you a number of options for alternative approaches that can give you a fighting chance of holding on to your audience's attention and enthusiasm. She only made one mistake... ten minutes is way too long a grace period for most listeners. I say they'll make up their mind about you in the first two or three minutes. So don't blow it! Read her article. Take notes. Change your presentations.
"Learn it. Know it. Live it."