This is your lizard brain taking over. The only way to get it to shut up is to practice until your lizard brain learns that the audience is not about to eat you. In the meantime, fake it 'til you make it.
So we have this natural instinct to be freaked out by unfamiliar situations. After all, who knows when a tiger will jump out and eat you?
Unfortunately, this part of our brain also makes us assume that the audience will consume us whole if we make a mistake or show weakness, which freaks us out, which makes us more likely to act like a nervous wreck. This is human nature -- just about everyone feels it in front of audiences. Some of us are just a bit better at hiding it (usually through practice).
The only way to quiet that horrible nagging voice in the back of your head is through practice. While you practice, there are a few things you can do to make your audience more receptive (and less scary looking) to make the process of practicing easier:
- Speak to your audience's interest
- Limit one-way speaking
- Rehearse your lines
Most of the people I see who are nervous speaking in front of others focus on what they need to say, instead of what the audience wants to hear. If you engage the audience, they are more likely to be interested, smile, lean forward, and otherwise give positive reinforcement that helps quell that unsettled feeling you have standing in front of them.
Try to make sure your presentation is actually interesting in content to the audience. One sure way to do that is to limit the amount of time you spend lecturing at them. People get bored listening to presentations (usually 15-20 minutes is the amount of time when people start nodding off), so keeping them engaged will mean that they take some of the attention, and give you breaks in between.
While Kate says that you can't make a script, I tend to disagree. You can make a script, and practice with the script (but not reading off it), and that will help make the story rote (even if you change up the words when you get in front of people). Having as many queues as you can, both on the slides and in the notes, will make it easier for you to focus on the act of giving the presentation, rather than the people you are giving it to.
I always write up notes, and practice for at least half the time it will consume for the audience (if I speak in front of 20 people for 20 minutes, I will practice for 20 x 20 / 2 = 200 minutes). This may seem like overkill, but when you've given the same presentation 10 times in a row, you definitely don't have trouble remembering what to say even if you discover halfway through the presentation that your fly is down.
Fake It 'Til You Make It
This was the best advice I ever got when it came to speaking in front of people. I used to be a wreck speaking in front of folks. New jobs, speaking in front of 5 people on my team would make me horribly nervous. In the comedy of errors that has been my professional career, I ended up with a job where 90% of my work was speaking in front of groups of 20-40 people daily.
I was tossed to the lions and was forced to sink or swim. So I did what I could and pretended that I was:
(I was neither)
But for whatever reason, playing the role of the confident public speaker was easier than trying to be the confident public speaker myself, and it made me more willing to get myself out there each time without entirely falling apart, and that got me better in the long run. Just imagine that you are playing the role of a famous public speaker. It won't fix everything, but it just may make the process of practice better.
Putting yourself out there time after time will breed real confidence, and competence. In the meantime, you may have to fake them, but we all start somewhere, right?
When you commit to doing it (even if you're faking the confidence), you may find out (as I did) that even if you have the worst speaking experience ever it isn't actually as bad as you imagined it was. Whenever I need to speak in public for something outside my comfort zone (speaking in a foreign language in front of 500 people on 3-day notice, for instance), I just fake it.