If you ever have to give product demonstrations, you'll be amused by Guy Kawasaki's Five Things Not To Do in a Demo on the Open Forum. However, many of these "things to avoid" also happen to be no-go areas in other parts of the marketing mix.
Not doing your homework when it comes to the audience's needs is a good example of something which doesn't just apply to demonstrations. If you're taking a product to an construction industry exhibition, you need to make sure the focus of your handouts is relevant to architects and builders, rather than it being a general list of features for everyone "in the hope that your customer will eventually see something of interest".
Another presentational no-no is not getting to the point quickly. We all know not to begin a talk by "putting things in context" with a corporate overview, so don't do it in your press releases either. Don't encourage your customers to wonder where your presentations are going, and similarly, don't wander all around the subject on your website, which should be a series of funnels through to your desired calls-to-action. And just as you shouldn't show the same demonstration, regardless of the audience's technical level, you should also avoid writing catch-all descriptions in your literature, and instead produce different explanations for different types of reader, each well-signposted of course.