Keep in mind that it's all about optimizing the resonance for the voice you've got. You shouldn't be trying to talk like James Earl Jones if your voice is naturally higher - it will hurt your vocal chords and you'll sound like you're trying to talk "low" which makes you sound insecure. You have to find where your voice naturally wants to be and make sure you're using it correctly - making it loud, forceful and resonant.
A big part of it is also slowing down the pace at which you speak. Most people talk way too fast. In my opinion, talking fast makes you seem less confident - it's as though you're rushing to get out what you want to say because you're afraid people will stop listening. People who talk slowly and deliberately give off a confidence that says "what I'm saying is interesting and I know you're going to hear me out."
You might want to listen to the pace at which hypnotists talk. They create a sense of steadiness and being in control just in their pacing.
A big part of it is also slowing down the pace at which you speak. Most people talk way too fast. In my opinion, talking fast makes you seem less confident - it's as though you're rushing to get out what you want to say because you're afraid people will stop listening. People who talk slowly and deliberately give off a confidence that says "what I'm saying is interesting and I know you're going to hear me out."
You might want to listen to the pace at which hypnotists talk. They create a sense of steadiness and being in control just in their pacing.