Even the best presenters rehearse. Rehearsing is important to get a sense of the length of your presentation, to order and program thoughts into your mind, and to sound out the most difficult parts and passages. New concepts and ideas are also discovered in rehearsal.
Do not practice in front of a mirror or record yourself unless your audience will be sitting two feet in front of you, or your presentation is for release as a video.
Ideal ways to rehearse:
Take a long walk in a secluded place (path in woods, along the beach) where you can speak out loud without disturbing anyone. You will be physically active, breathing, and addressing the broadest possible audience (the world).
Practice on your pets. See if you can get and hold their attention with your voice and gesture. If you don’t have pets, line up your stuffed animals or action figures as an audience and address your presentation to these avatars.
Put on some favorite music in your office/living/family/bedroom that is in keeping with the theme of your presentation and practice your presentation as if your words were lyrics to the music.