If you have not heard of Scopial T-shirts, its a website where you can buy cool t-shirts. If you are not into buying, you could submit t-shirts designs and win prize money.
This is one of those site that you say why didn't i think of that first. Since some one else has started it, here are some ideas to improve.
They should do themed tshirts like new year shirts and put a dead line on up to when people can buy it. Also football world cup is coming do a single or couple of t-shirts on that. Or Sachin's 20 years etc.
Also do advertising giving unique urls, its easy to do with url shortening services. Once a user gets a certain amount of visitors to that particular page he gets a free t-shirt.
Also try to bring the price down to 250 Rs range or even cheaper where it could become an impulse buy.