My best tip for public speaking: speak loudly. Don't scream, but it makes you stand out and seem incredibly confident if you're just speaking louder (more loudly?) than everyone else. This, of course, takes a certain amount of confidence to do.
Personally I'd recommend "deeper" (if possible) and "more relaxed" before "louder." At least, that is what you want for an English-speaking audience. If you're loud but your voice is nasal or shrill then you may have the opposite effect.
A deep, relaxed, resonant voice conveys the assurance and confidence you're looking for. People associate it with maturity, wisdom, and masculinity (Think Morgan Freeman or James Earl Jones). People associate a shallow, shrill, nasal voice with a whining child. Most people can also recognize tension unconsciously.
The key to speaking with resonance is slow, relaxed, deep breathing (sort of like Yoga breathing). This has the nice side effect of helping you stay relaxed.
This is very true -- I always have an amazing tendency to talk faster than normal when presenting, and I find that taking the time to calm yourself before talking is effective. But you can always seem to catch people's eccentricities too; I always seem to end up scratching my right ear while talking, and have no idea I was doing it until people ask why my ear was so itchy.
I'll try deeper tones in the future. Darth Vader gets respect, period.
The word my vocal directors always used for what I think you mean is "project." (As in "project your voice to the back of the auditorium") A quick google turned up:
