No, this is not anything to do with superstition, it's to do with rehearsal. Yes, you have to rehearse, but never, never stand in front of a mirror to practice your speech. I know that many presentation experts will tell you that mirror rehearsals are a good idea. However, as a regular reader of these tips, you will know that I don't always agree with conventional wisdom.
Allow me to explain. When you speak to a mirror, you are trying to do two things at the same time - assess your speech at the same time as delivering it. That puts you in the odd position of trying to be both audience and presenter. Even worse, if you follow the good practice of making eye contact with your audience, you won't see someone reacting to your ideas, you will see yourself trying to remember your lines. All in all, it's a waste of time.
What about videoing yourself? Well, that can work, but it can lead to focusing on small gestures, such as how often you scratch your nose, which would pass unnoticed in front of a live audience. Also, people tend to behave differently in front of a camera.
What's the answer? You guessed it. Find someone to rehearse with. It should be someone you trust to give you honest feedback. You don't need to take up too much of their time, since one or two rehearsals are sufficient. You want to sound fresh when you speak. If you are in a hotel room, on your own before a speech, you can rehearse your lines over the phone to a friend. But please, not the mirror.