Some time ago Threadless introduced the Threadless Thriftee promotion which gives you the opportunity to get a shirt for just $10 bucks. A random sized short sleeved is displayed and available for $10 and once it is purchased a new random shirt is displayed. This process keeps on repeating. You can watch these shirts over on the Thriftee page or if you are looking for a certain shirt size you can subscribe to the Thriftee RSS feed.
If you are looking for a certain shirt there is a great Thriftee Alert System over at Loves Threadless. Just enter your email, AIM ID, or cell phone number and they will alert you when a particular shirt and style in your size is available. Currently Loves Threadless has over 200 users using the system and over 600 alert requests. Be careful how broad your request is (i.e. just by size) and you might be getting a lot of messages.