A friend asked me last week whether it's a good idea to practice in front of a mirror before delivering a speech or presentation. The answer to the first half of his question is a definitive yes; one should always try to find time to practice out loud beforehand. It's the single best way to keep nervousness at bay, and it greatly improves pacing and delivery.
But I don't recommend the mirror part. It's distracting. Kind of like steering with a tiller for the first time - it looks and feels "backwards," and you're bound to be self-conscious.
Better to grab a video camera or your smartphone and record yourself. If you don't have a tripod, just balance the recording device on a stack of books or lean it on a shelf. This way, you can concentrate on your delivery and visualize the environment and the audience. Deliver your entire presentation if you have time, then play it back and take notes. Are you gesturing in a strong but non-distracting way? Are you emphasizing key words? Are making steady eye contact with the (imaginary) audience, or are you looking down at your notes too much? Try to practice and review the entire thing at least three times before you do it for real. There's nothing like seeing what the audience will see before they see it.