In order to sell more t-shirts you have to Stay Active. By promoting at local events and through social media you can generate more sells, it's really that simple. It's like owning a small coffee shop in a hip town, the more active you are in your community the more the people of that community want to support your coffee shop.
People love to feel like they have a sense of belonging, so why not make them feel that way by purchasing your t-shirts? You can accomplish this building a community and managing it daily.