Watch this: http://www.youtube.com/watch?v=Fjim0ooJGdg
It’s Ricky Gervais at the 2008 Emmy awards. Notice how at 2 minutes and 8 seconds, he apparently goes off-script and verbally attacks Steve Carrell. It’s a fantastic example of how doing something seemingly unprepared creates an atmosphere of compelling uncertainty. At an event where everything is strictly regimented and unsurprising, you can be sure that everyone in the audience was absolutely focused on whatever Mr. Gervais would do next. Now OK, it was obviously completely scripted and well rehearsed, but it felt dangerous.
Another technique is to get out your script, look at it and then just throw it away. The audience thinks “Wow”, they’re giving us something completely unprepared”, but it’s really all a load of old pretendey. That’s a great device where the occasion has genuine importance, such as a eulogy, where the audience doesn’t just want to hear what you know, but what you feel.
If you’re really good at improvising, then fine, but for most people (who aren’t), try tricking the audience into thinking you’re giving them some impromptu stuff; you can be sure they’ll pay you complete attention. Which is a very good thing indeed, especially when it’s just before lunch.