How to gain self confidence

Today is one of those days when your group has to make a presentation to a client. This is an important occasion because it is an opportunity to get your boss' attention. It could mean a raise or a promotion if you could just muster the courage to stand there in front of these people and present your proposal. The problem is, your shyness gets the better of you, and you are relegated to the background.
You sat there mesmerized, as your colleague Elena made a winning presentation of your proposal. She stood there, dressed in a simple gray suit that spelled confidence! "Why can't I gather enough courage to present my work to these people when I know this project like the back of my hand," you ask yourself.
Self confidence. This is what you lack, and this is what your colleague Elena has. But what is self confidence? Is it the ability to speak in a loud voice so you can get people's attention? Is it about power dressing?
Self confidence is an attitude that is characterized by a positive belief that one can take control of one's life and of one's plans.