Women: How did you find your "big girl voice" in the workplace?

I’m a 24 year old young professional recently returned to NYC from a South American hiatus to start my dream job. I love it! Aaaand I’m pretty good at it if I say so myself. However, I have one small problem.

Sometimes I tend to be a little shy, particularly around my bosses whom I admire and respect. Our workplace is pretty egalitarian with a laid back attitude but I’m in serious newbie mode and am always afraid of stepping on toes. (As I should be while still on three months probation.)

How do I become more confident and learn to speak up more? Do you have any secrets? Is it something you wear or something you practice that helps you be more assertive (not aggressive) at work? I love this job and really want to do my best but it’s embarrassing when my boss has to remind me to “speak up, damnit!” :p