We're going to try and utilise the fact you can 'create' a tshirt with just a link, i.e. http://helvetitee.com/tee/foo-bar. It's so quick to react to trends and happenings, or suggest a tee to someone based on their actions, that we feel the product can benefit from virality and p2p sharing. Plus a healthy mix of the usual Google ads etc.
Btw, how do you plan to do marketing on this? (Besides posting it on Hacker News - which probably brought you a ton of traffic already)