Don't anticipate the worst and go into it with the attitude that you just want to get it over with. Make sure the set-up is to your satisfaction. It's amazing how often most speakers fail to deliver on that simple demand. Of course not. I'll have to admit, I'm bad about this.
Some speakers swear by videotaping their practice sessions or speaking before a mirror.
That's never worked for me, but I wouldn't discourage you from trying it since I may be an exception to the rule. It really works. In other words, if you are speaking for 20 minutes, you should invest 20 hours in research, development, organizing, outlining, fleshing out, and rehearsing your presentation. It is, but it's necessary if you want to delivery a dynamite presentation. Most people have no idea how much time is required to put together a presentation or seminar.