For those who've been to fairs and congresses, you've seen convincing demos as well as very poor ones. And if you've done one yourself, you also know about the "demo effect", killing all credibility because a silly thing makes the whole thing unworkable.
Well an effective demo needs to be prepared properly. And be simple to avoid the bad demo effect. This requires much more time than you'd expect!
Think about what you're trying to achieve, more in terms of touching your audience rather than trying to over impress them with details. Create a story and communicate an emotion they'll remember, it's as simple as that.