I love tee shirts, and have been trying to break into tee-shirt design for some time now. I've submitted to various companies, including (but not limited to) Threadless, Design By Human, Goodjoe, Teefury, Qwertee, Other Tees, etc... Sometimes my designs make it into voting. Other times, not. I have yet to be printed by one of these companies.
To offset that, I post work on Zazzle and Society6. I admit, some of my designs don't quite fit what they are promoting. Other times, it's frustrating because even tho I was on topic with a pop culture mash-up, for example, it wasn't the right one. Well, if you expect to get anywhere, you will receive rejections. Learn what you can from any useful notes, and keep doing what you love. I keep on doing stuff, and hopefully will have a new collection of designs to inflict, er, share with the world!
Here are some of my designs:
To find some of these shirts, in these forms or altered, check out some of my links...
on Zazzle: http://zazzle.com/korokstudios
on Society6: http://society6.com/KoreyWatkins
on Red Bubble: http://www.redbubble.com/people/korokstudios/shop
on Cafe Press: http://www.cafepress.com/korokstudios