When the player feels the character has accomplished enough to warrant improving in some trait (and he feels he’s been role-playing well), he petitions the GM for permission to raise it. A trait can only be raised one level at a time. A trait must be used more to raise it from Good to Great than Fair to Good, and so on. It should be easier to raise a Skill than an attribute.
Or the GM can simply award an improvement in a trait she feels deserves to be raised. In these cases, there is never a corresponding reduction of another trait this is character development, not creation.