1. Know your subject. This is usually the easy one. Know exactly how you came to your conclusions, and why it's valid. You can defend the hell out of it if you need to, and you can honestly engage with useful criticism.
2. Know what you're going to say. This comes from practice. Are you worried about time? Practice and time yourself. Are you worried about how you look? Practice and video it and watch it. Are you worried about sounding stupid? Practice and record and listen to it. Get someone else to listen to it.
3. Know your audience. This is the hardest one. It comes from experience. What does your audience care about? How can you move your audience from where they are to where you want them to be? Get to know them. Become familiar with who they are. This is why talking with your friends is easy, but an auditorium of strangers is scary. This is also why making eye contact with different audience members is helpful. You need to respect your audience, and also believe that your audience respects you.
This was a bit of a brain dump (my best write-up has been lost to /dev/null, unfortunately), but hopefully you get the idea. I suspect this would serve as a good core to learning public speaking, though.