Since college, I've been involved either with the teaching profession or the military, so I've seen public speaking, up-close, on a pretty much everyday basis. I've spoken in public, been spoken to in public, taken courses on speaking in public, prepared briefs in front of a peer audience, helped peers prepare their briefs, etc. Through that time, I've seen the good, the bad, and the ugly. And here's my one piece of advice to any would-be public speaker: project your voice. Whether you're a classroom teacher trying to command discipline and respect, or whether you're a military officer concerned with your bearing and your command presence, the one thing you can do to instantly improve your ability to meet your goal is to raise the volume and enunciate.
Before you work through some kind of workshop-inspired checklist about eye contact with the audience, memorizing your opener, visualization, well-timed pauses, etc. just remember that a person who stands in front of a room and speaks loudly and clearly appears more confident, more knowledgeable, and more charismatic. Even if you have to be *too* loud (that is, louder than you would naturally think is appropriate), that's probably okay, as it means you'll naturally regress downwards a few decibels as you go on. The same thing applies for enunciation -- even if you have to *over-enunciate* while you practice, it'll help you on gameday.
If you still don't believe me, just wait until the next time you have to sit through a day's worth of briefs, presentations, classes or whatever it is that you do in your field. When you see someone mumbling or whispering in front of an audience, do a quick sweep to gauge the rest of the room. Trust me, you're not the only one in pain. Now do the same thing when someone who can project is up at bat. Just by the body language of the people in the room alone, you should be able to quickly see the night-and-day difference.