You've gotten a lot of replies already but I figured I might as well add one more to the mix.
I used to be a lot more quiet in early high school. I still am, but I feel a lot more comfortable coming out of my shell when the situation calls for it (interviews, certain workplace environments, etc.). I think I was mostly able to achieve this by seeking leadership opportunities in clubs I excelled in. This wasn't really on purpose to be honest - it was orchestra in high school and dance in college, but doing well gave people a reason to talk and listen to me, and that really helped me overcome a lot of my social anxiety.
I know that's easier said than done though. Also this has only worked for one person that I know of so far (me), so take it with a grain of salt.