A presentation is physical. You need physical energy supported by plenty of oxygen to sustain a compelling performance.
Get exercise in the days leading up to your presentation (its always a good idea to get exercise!) The best form of exercise for presentations is walking. Take long walks on a treadmill or in the wild. As you walk, breathe in for a count of three, hold your breath for a count of three, and then breath out counting out loud one, two, three. Hold two, three and then repeat. This will build up your capacity to take in oxygen and sustain the breath you need to support vocal energy (Also see Rehearse.)
Hold your head up and address the world as you walk, as if the world is your audience. Meet the world head on, do not retreat into a mp3 induced bubble.
Exercise will not only build up your energy, wind, and stamina. It will also help relax you.