I realize I just told you all about how to measure yourself for a bra. But really, the very best way to make sure your bra fits is to go to an experienced fitter.
Not only are they skilled experts in ascertaining the fit of your bra, but they can tell you what styles might work best for you based on the shape and placement of your breasts, what your needs are, etc. They should also know their stock well, and, just like clothes, fit will vary from brand to brand and style to style. So they will be able to tell you, “you would normally be a 32F but this brand runs loose in the band, so I would suggest that you try a 30FF.” And it is invaluable to be able to try things on and see which ones you like the fit of the best. They should also show you how to put on a bra correctly. This can make a HUGE difference in the size you need. In addition, if you have been wearing a style that's vastly wrong for you, then you won't be used to the feeling of a well fitting bra and so anything that's better than what you have could seem perfect at first!
However, sadly, an experienced bra fitter is hard to find, especially in the US. I have heard all too many stories of women going to an “experienced bra fitter” only to have them try to convince the woman that they need the wrong size, just so they can make a sale, or because they weren't so experienced on fit after all.
This is why I told you to measure yourself. You have to go informed. See what size you think you need before you go to try on bras. Be informed on how a properly fitting bra should look.
It also might be the case that you simply cannot get to a fitter. If this is the case, I encourage you to make this a priority. However, in the meantime, you may wish to measure yourself and order online. Even if the size you get isn't perfect for you, it will probably be a big improvement!
I would suggest you research online for the best fitters in your area. If you travel, make it a habit to look and see if there are any well reviewed or recommended fitters in the place you are traveling to. If there are no fitters in your home area then this may be the only way you can get a fitting. If there are, then a second opinion never hurts anything!