I got my Underworks tri-top binder in the mail today and wow, I'm really impressed by the results! For sizing I did what's been recommended here (averaging your bust and under bust measurements) and from that ordered a size small. I think a medium would have worked better because I couldn't get it on at first but after some major stretching out of the small I can get it on and off just fine!
Just thought I'd add what my experience with sizing was since I see questions about that getting asked a lot here :3
Yui Hirasawa (K-On!)
Ai Mikaze (Uta no Prince Sama)
Anri Sonohara (Durarara!!)
Anime Detour 2013