A number of people here have provided some interesting public speaking anecdotes, and have stressed the importance of practicing aloud. I couldn’t agree with that more. Beyond that, I also recommend that you tape-record yourself or even practice in front of someone that can give you objective feedback (and not someone that will just nod their head and agree with you all the time – you really want to find a friend that knows what they’re doing and can pay attention to you). Practicing in front of a mirror without hearing how you really sound or without getting any feedback doesn’t do you that much good; a lot of the time, how you think you sound can be very different from how you come across.
In addition to practicing aloud and recording yourself, you want to put yourself in real-life situations that require you to polish your presentation skills. Toastmasters is one way that a number of people I know sign up for. Personally, I’ve never done Toastmasters, but at a younger age, I myself was not a very gifted public speaker. However, rather than allowing myself to be paralyzed by my fear of speaking, I seized the opportunity to find situations that would force me to step out of my comfort zone. Whether it’s giving presentations to people, being a youth mentor, serving on the leadership committee of a local board or alumni chapter, these are all situations external from work where you can improve your speech abilities. And you will feel compelled to work hard, because even though you can’t typically get fired or released from these roles, the responsibilities of being a good mentor or community leader will most likely drive you to improve yourself.
I know there’s a great deal of focus on this board on CFA, MBA, resumes, etc…all of that stuff helps you get your foot in the door of an industry, but making an impression on your colleagues or prospective employers comes down to your personal presentation. Even in the most competitive jobs, there are enough people that look like they could do the job based on how they look on paper; ultimately, who gets hired or promoted often has to do with how you present yourself and how you captivate the attention of the people around you.
As such, I believe that public speaking is one of the most important skills you can have in any industry (normally I would have said “leadership,” but that’s really more of a personal quality or attribute than a skill). Hope this helps!