Expressing anarchism in a general sense has little to do with clothing choices. It's not totally unrelated, tho. Recently, i have been seeing folks at Occupy events wearing black bandannas around their necks. I assume this is an explicit identification as anarchist; especially i assume that they're distinguishing themselves from the people wearing red bandannas (socialists, presumably). Wearing circle-A apparel might be one way to express anarchism, but as with any symbol it might be interpreted differently by others from what it means to you.
To express anarchy you must come to an understanding of it first. What does anarchy mean to you? What experiences have you had that have led you to anarchism? What particular messages about anarchy do you want to convey? What have others said about anarchism? What experiences and realizations have other anarchists had that might inform your own understanding? You aren't a poser if you know what you're talking about.
Once you are clear about where you are coming from, you can attempt to communicate it to others. How you express your anarchism is entirely up to you. You could do screenprints, surrealist street theatre, direct actions, anti-oppression workshops, a local newsletter, or any number of other things. My advice is to try a bunch of things, see what you are good at and what you have a passion for.