In the Navy Seals, they have a saying, the more you sweat in peacetime, the less you bleed in war. Applied to real life- if you have to face a battle, it pays to stage an even larger, scarier one in advance. When the big battle arrives, your confidence will be higher because you already survived the previous endeavour.
When I was building up confidence to become a motivational speaker, this meant every speech I gave was a stepping stone to the next one. If I had a challenging audience, or just a new audience I wasn't familiar with, I would remind myself that the next time I have this audience or this speaking experience, it will be easier.
To transition to be a funny motivational speaker, I practiced stand- up comedy. Again, everytime I bombed ( which i did alot), I would remind myself that the next time this will be easier... and it always was.
As a true, learn by doing entrepreneur, I have learned the hard way that practice makes perfect. The more prepared I am, the better a motivational speech will be for the audience and for me.
Applied to work, it helps to practice with people who are better than you, to keep your skills sharp. If you have to make a presentation in front of a group that scares you, first deliver a talk in front of a group that scares you more.