Sure, self expression sometimes ties into your image - which is what clothes, piercings, and tattoos are to me - but it can never be solely reliant on that.
Self expression is a highly personalized thing. Each person has an expressive outlet, and shows it in their own ways, via radio shows, via tv personalities, blogging, painting, parenting, cleaning, hell, even the idea of how a person moves can be a form of self expression, and often a person expresses themselves differently in a multitude of settings, professionally, familiarly, and in the home.
Consider some of the greatest actors and their roles. Notice how they move. Notice how they talk, their dialect, their expressions. All of these are tied into an expression of that person's self.
Now think about this - they say that every book a person writes carries a level of self-expression with it. If you have read mass quantities of a specific author then you may have come across some specific "tones" that read in that author's books. Now I'm not meaning themes or genres, we all know that Stephen King is a master of thrillers, but when you read through his pieces you start to understand the way he writes, his process. If you threw in a different author - for example Laurel K Hamilton who in her own way has some wonderful thrillers out there as well, you would immediately see a different style of writing, a different expression of self.
The best books are built with strong characters - characters that have their own self to express. So the question becomes how do you show your character as an individual separate from yourself. An individual with thought patterns different from your own, and stay true to that character while still showing your own artistic voice?
My new piece is written from a barely legal blond haired beautiful 18 year old. The type of girl that gets anything she wants because she's so beautiful. I am an attractive 30 year old woman, but I certainly have little experience with getting everything I want because of my looks. Entering her mind was a fun experience, and I hope it is for my readers as well.
Shelby is fun, sexy, and exuberant, her forms of self-expression are vastly different from my own, but I feel the book reads well, coming across cleanly and believably. I hope my readers feel the same.