Be yourself, but always be challenging yourself and transforming yourself into who you want to be. Don't pretend that you are someone that you're not, but develop the qualities in yourself that you want to improve on.
For instance, let's say you want to be a popular person. It would be silly for you to go out and act like you know everyone and that everyone knows you..it'd be pretentious. But if you start developing skills on meeting people and maintaining relationships, then you'll start to transform into someone popular.