I've assembled some speaking tips from some local sources who seem to know what they're doing. They practiced what they preached when I observed them live. Here ya go.
Speak using solid tonal support. This seems to involve using muscles in the lower abdominal region and lower back area, primarily the diaphragm. Tenors carry farther in public than baritones by using these muscles. I'm not a singer but I do enjoy yelling at stupid people, so I'll tighten my abdomen the next time I yell at a moron to ensure other people hanging around hear every insult I hurl at my hapless target.
Raising your pitch from 100 cycles/second to to 200 cycles/second makes public speaking more effective. Serious public speakers probably use a spectrum analyzer (aka frequency analyzer) to measure their cycles. I don't need no stinkin' analyzer.
Use your "outdoor voice" to raise your vocal intensity and pressure. This may seem like common sense, but I look at politicians for confirmation. The winners of elections do well in televised debates because they use their outdoor voices even while inside with microphones.
Keep your head up and face available so the audience can match your words to your facial expressions, and talk to the people in the back of the room so everyone can hear you. That's probably good advice for candidates stumping for votes who have to speak unamplified in high school gymnasiums and hotel conference rooms.
Shorter points are better, probably because most people are too dumb to follow complicated arguments. This isn't ancient Greece or Rome where educated people came to public forums just to hear philosophers expound on their pet theories for hours. This is America, where the calorie-addicted lumpen proletariat has been trained to think in sound bites that expire before their sugar rush wears off.
Simple repetition is memorable and effective. Keep hammering away at your main idea until even the dullest people can remember it and are ready to join your movement. Beethoven's Fifth Symphony repeated four notes in seven minutes. Martin Luther King Jr.'s "I Have a Dream" speech repeated four words nine times in seven minutes. I think 4x7 is some kind of gold standard in making humans pay attention. I'll try that by repeating four words, "you are totally stupid," seven times to the next person I meet at a high-end cocktail party. BTW, MLK also sprinkled that speech with visual references that cued his listeners to imagine inspiration scenes. Human beings are visual and need to be handed pretty pictures to help them understand things.
Those voice lessons are important but I don't think they're the total package. Illusionists are known to distract audiences with sleight-of-hand while pretending to pull rabbits out of hats. Successful leaders can use gestures and posture just as effectively. The discipline known as neuro-linguistic programming (NLP) is one approach to defining and codifying non-verbal cues that alter behavior. I've tried various hand gestures and body stances from NLP and other sources with varying degrees of effect. I discovered years ago that posturing techniques such as mirroring have no effect on sociopaths because they are incapable of feeling empathy. NLP probably works best in front of large crowds or on broadcast media because lots of empathetic suckers will be watching.
I will continue to broaden my knowledge of speech, gesture, and posture because I love impressing people with my genius. My success in employing these techniques may very well determine the course of human history. You people will be glad when you put me in charge. I promise that world domination under the Alfidi regime will be benevolent. Just listen to my vocal intonation and watch my thumb and forefinger to know that you can trust what I say.