People are noticeable when they are talking, people are not noticeable when they are quiet. It depends on your state of mind while being quiet, or loud, as well. I have a large energy, so I generally stand out to people even when I am quiet; but I am rarely the focus of a group because I simply don't talk much.
When I am passionate about something, I can talk alot - while I am talking the words seem worth while and also appear to be interesting to other people.
You are either quiet and timid about talking, quiet and confident about talking, loud and timid about talking, or loud and confident about talking.
Not sure how you can train that though (I have been like that since I can remember) - mostly just with affirmations I would think, and practicing not saying much around people. Don't be surprised though if you are not the focus of people's attention.