One of the fears people have about public speaking is the fear of being seen as nervous while in front of an audience. People believe that the audience will see their knees knocking, or that their nerves will get in the way of delivering a great presentation.
Let's consider this: the speaker, with nerves of steel, and having delivered his presentation many, many times, comes out in front of the audience exuding calm. He or she runs through the presentation with nary a glitch and a steady voice. Is this where you want to be?
Maybe. Sort of. Kinda.
A calm and steady speaker with absolutely no edge can certainly deliver. But is he/she delivering in a way that is too calm? Too steady? Too indicative of a rote performance?
Does the audience sense that he/she is essentially "phoning it in"? Does the calm lead or equate to a lack of enthusiasm and even monotony?
Either of these can cause the audience to turn away from the speaker, to lose interest, and even, dare I say, be lulled into the semi-hypnotic state that monotonous presenters seem to engender in their audiences.
Being at least a little nervous is actually a good thing. Nervous energy is energy. Nervousness can give you a boost of adrenaline. Use that energy in your performance and the audience will respond.
As someone who speaks professionally and entertains professionally (as a psychic entertainer/mentalist and former magician), I am more concerned before a presentation or show if I am not at least a little nervous. That means I'm not really on my game and I may never grab the audience's attention in the ways that I want.
Knees knocking? Move around (if not tied to a podium).
Notice your voice cracking? Start playing with vocal variety (and/or take a sip of water...maybe you have a little dry mouth).
Know that most people are nervous about speaking (if not downright afraid). If they notice a little of that in you, it actually connects them to you while thinking "Wow, I'd be so much more nervous than that!"
So take your nerves as a good sign that you are aware of yourself and will be more aware of your audience. Channel the energy into enthusiasm and you'll be ahead of the game every time.
Or go for utter calm and take the chance that you'll calm the audience as well...until they fall asleep.