It's ok to be shy. But if you want to change it, get a bit radical about it.
Really just step up to the plate and pretend like you don't care how people perceive you, and that you're completely comfortable being who you are (which is how we should all be). Sales people are often the most confident on the outside stereotype we first think of, but I can tell you, from experience, that they're often the most insecure. If they can seem confident, anyone can.