Be ready for the demo, and prepare your audience at the outset. Of course, you should have your slides working and your demo up and running already. And you need to be familiar with the features of the tools you are using for the demo (highlights, pointers, ...). The kind of preparation Peter suggest is right at the start of the webinar. Use the interactive features of the webinar tool to ask people if they can see the top right and bottom left of your screen (with a mouse wiggle). Make sure people can hear. Make sure you can hear people (in small enough audiences), and make sure you can mute people. Ask people to tell you who they are -- in the case of this demo, he asked people to type their title into the Q&A panel. Not only does this make sure everything is working with the technology, but it also brings people into the discussion right away.