How good are your bosses, really? Are they helping you to get ready for this or do they expect you to get yourself ready? Would they take the time to let you run through your presentation in front of them so that they can critique your style and help think through a few questions or scenarios? Failing that, can you gather an audience of your knowledgeable peers for the same purpose?
If this was a flag officer visiting/touring a training command, I would've been mustered for daily practice briefings starting about two weeks ago. I'd also probably be on rev 14 of the brief by now.
The guy has probably seen a gazillion presentations and can listen while reading. If you have a habit of reading from your slides... don't.
If you've gone over the presentation so many times that you know it by heart and can race through it at 200 words per minute... don't.
President of a Berkshire Hathaway owned company.
If you're saying that the president is running a really really big company with lots and lots of people, then he'll either be an impatient fire-breather or a really nice guy. If he's a fire-breather and doesn't like what you're saying then he'll flame on you or your bosses. If he flames on you then try to let the warmth wash over your body, don't take it personally, answer the questions you can answer, and say that you'll work on fixing [whatever it was he didn't like]. He didn't show up there specifically to execute you and his temper tantrum may just be a way of emphasizing his [-]enthusiasm[/-] point. Hopefully your bosses will be thankful that you're taking the heat, not them, and they'll think of you more kindly at performance-review time. Of course a really good boss would jump in to defend and help out.
If this president is a really nice guy then he'll make you feel as if you're the most important briefer he's ever listened to, and you'll wonder why you ever felt nervous about it.
If you have really good bosses then they'll jump in when he asks a complex question (or if you overlook a mistake) and clarify "Well, what JB meant to say was..."
Like Martha says, the better you know your stuff then the more you'll be able to talk in a normal voice without that high-pitched quaver choking up your throat. It'd be bonus if you could sneak a water bottle up to your podium for an occasional swig.