Honestly... putting myself in situations where I HAVE to speak clearly has helped me a lot. I got a job out of college working in a loud manufacturing setting (a paper mill), and I basically spent at least an hour each day half-shouting to people on the floor to offer them advice, ask a question, or just shoot the breeze. And honestly, the workers seemed a shade put off at first, but they all really respected me for being willing to chat with them even if I was uncomfortable. And then it's little stuff that I never used to do on my own: order at a fast food restaurant (still can't do drive-thrus, but we'll get there!), making doctor's appointments by phone, etc. I still sweat profusely after 2 minutes of a phone interview, but putting myself out there has definitely helped.