True & Co., an online bra shop, has a good system: They ask you some Qs about bra fit, and they send you a sample box with a few sizes to try. You buy the one that works. (This sounds way more pleasant to me than the "fitting appointments" I've been through, which are awkward at best.) The thing about bras is, you can "know" that you're a 34B or whatever, but each one seems to fit differently! Cup and band size are such small pieces of the boob puzzle! So a box of mixed sizes delivered to my home? Yes, please. Also, founder Michelle Lam says that True & Co.'s research has identified 6,000 distinct female body types. 6K! This company does not take short-cuts.
The bras aren't cheap -- they start around $40 -- but honestly having 2-3 great ones that fit really well is so much better than having 10 that pinch or fall off or make your shirts lumpy. This I know from experience.
So take the quiz, await your bra samples, and try one on for yourself.