Taking part in the blog community may be a good way to get additional traffic. Make your own store blog/news site and be active in discussions on other related sites. As long as it doesn't come off as pure self promotion, and you are taking part and contributing to the discussion, this might be a good tool. I doesn't have to cost you much, but It will probably take up some time.
You can also advertise on a lot of blogs these days. Take a look at
Blogads, Adbrite and such- you can also look around to see what sorts of advertising the different sites run, or contact their owners.
Another tip is to get a page on
MySpace, as a lot of people are using this resource. I see a lot of t-shirt designers and retailers are using this as a channel for marketing.
It is all about creating a buzz for you biz.