Selling t-shirts online is getting harder and harder because of the growing competition. T-shirt artworks matter (of course!) but you need a further push to succeed. Being able to generate word of mouth is the first element that comes into my mind. But the key is to have a good website where the click&buy; button is right there where it can trigger consumers' compulsory shopping instinct.
Check out the Rated Rookies website. It's a t-shirt collection showcase, nothing more but the point is that everything is simple and nicely done.