The logic behind this advice is that by rehearsing with a mirror, you’ll be able to see yourself in real time, gauge your effectiveness, and make instant adjustments.
If you’ve tried it and it works for you, go for it. But for most people, I can’t help thinking that practicing in front of a mirror serves as more of a distraction—one that makes you focus on your smallest facial expressions and gestures while taking your eye away from the larger picture.
It reminds me of another piece of public speaking “wisdom” that encourages you to reduce your fear by visualizing the audience members naked. (Given that the average business audience isn’t made up of people built like Matthew McConaughey or Heidi Klum, I’ve always found that advice more off-putting than helpful.)
So, how should you practice?
A video camera is a much more “real-life” gauge of how you’re doing as a speaker. Whereas a mirror presents a “close-up” that no audience will ever see, a video camera is a much better proxy for how the audience will actually see you (from a distance of several feet, not inches).