Want to be a more confident speaker? Pursue more awareness.
There are two kinds of confidence: 1) the kind you feel and 2) the kind that is an impression you make upon your listeners. It is possible to have #2 without #1 because the impression you make on others is not based on how you feel but rather on how you behave. That's where we'll start.
How do you behave confidently when you don't feel confident? It boils largely down to control – the ability to do what you want when you want to do it even if you're nervous. How do you develop that control? Practice, right? Well, kind of.
To have more control, become more aware.
I was a young teen when I first ski'd an expert run. I stood atop the steep, snowy slope looking down and feeling panicked. Images of me wiping out clouded my mind. But my older brother coached me to pick a line to ski and a point down the hill where I would make my first turn and to focus on executing the turn the best way I knew how. He was right. When I turned my awareness out of my head onto the slope, onto what I wanted to do with my body and what I was actually doing, I had the control I needed to get down the mountain unscathed.
The same is true of speaking. If you turn your awareness out of your head, onto your listeners, onto what you want to do with your voice and body and what you are actually doing, you'll increase your control. Knowing exactly what you want to do with your voice and body is where practice comes in. By practice I don't mean rehearsing your content; I mean experimenting with the outward act of speaking while raising your awareness of the behaviors you produce when you are speaking well.
Develop your concentration.
Being aware of your listeners, what you want to do and what you are actually doing – that's a lot to be aware of. Most of us were never taught to be that aware while we speak, so it can be both shocking and challenging when you first try it. But through repetition you'll adapt and ultimately, not only learn how to raise all that awareness but also how to keep it up over time. Simply put, you'll learn how to concentrate as you speak.
Here's the really good news: that control you develop and the outward impression of confidence you make will ultimately lead back to an inner feeling of confidence. Each instance of control will give you more reason to trust that you can do it again and again. And that's worth the practice.