Tech Squares periodically orders T-shirts. See T-shirt orders for information on recent orders.
Currently (Spring 2015), we have been giving t-shirts away to MIT students who attend intro night or the first night of class. We think this is good publicity, but it can also be a big expense. When designing and ordering t-shirts for this, try to keep the cost down, while making the t-shirts something students would want to wear. The approach I (Veronica) took in 2014 and 2015 was to use higher quality (softer) t-shirts as the base, figuring that this would make people wear them more. Also, t-shirts for students should be interesting, so consider doing a diagram of a call and not just the Tech Squares logo.
We sell t-shirts to club members and other interested people; generally for what it cost us.
It's been recommended (2015) that we use QRSTS in the future. Whatever vendor you use, look at how their pricing works in terms of printing or front & back and how many colors are printed before you choose how to set up your design.