Neither of the two above mentioned bra-measuring methods work for me! My ribs are 29, bust is 34.5. With timesaway's method I would be between a 34A and a 34B, but A is way too small for me and B cups sometimes cut into the sides of my boobs. But if I use aHikaru's method and take 34.5-29 I get a D or E cup, which is way too big for me!
The most accurate method for my body involves measuring the ribs, bust, and directly ABOVE the bust, which for me is 31.5. When i plug my numbers into this calculator http://www.85b.org/bra_calc.php it gives me a 34C, and I think i am currently between a B and a C.