I once spent a week stranded in Las Vegas, sleeping on someone's couch while waiting for my car to be repaired. While there, I learned about the proper fitting of bras. It turns out that the person who's apartment I was camped out in had worked for Dillard's for some time and knew a few things about it, and after a week, we were running out of things to talk about.
She told me that if you must wear a bra, as most of us with anything over a B cup must, then get a fitting at least once. She said to be prepared for surprising results. Then she told me the basics on doing it yourself.
She informed me that the most important issue when fitting a bra is how the cup fits. Forget everything else. Until you have the cup the right size, you can't continue with any degree of accuracy. When figuring your cup size, the basic method of measuring your barrel, around the ribs without including the breasts, then measuring with the breasts and taking the difference, subtracting 5 or 6 inches so you have an even number, and then dividing by 2 and converting the resultant number to a letter, 1 = A, 2 = B, 3 = C, etc, is a good place to start. Use a medium style bra, I prefer soft cup to underwire but that is a separate issue, not anything sexy or puritanical for fit issues. You can branch out to taste after you know what you need.
After you've found your proper cup size, then it's time to find out how big around it should be. This measurement is important, not for how the bra fits around your body so much as for how much strap comes between the cups themselves. I have the terrible misfortune to have a wide back. This means that if I buy a bra that fits properly around my body, the cups are set much too far apart. This is the difficulty that causes the front strap of the bra to be held away from the sternum. I was told to mostly ignore the fit of the bra around the body as that can always be adjusted with a little sewing or extenders, and that the same goes for the shoulder straps. I am personally of the opinion that all shoulder straps should be detachable at the back, so that if necessary, you can cross them, but most manufacturers haven't followed through on this yet.
I personally feel very sorry for the women with large breasts and small frames. I have only a C cup and I have difficulty finding one that is small enough around with a large enough cup size, and while the back can be taken in, it's much more difficult to adjust the distance between the cups. Thankfully, the current trend seems to be an expansion of available sizes and size combinations. Now if only I could find some fun leopard print ones in soft cup.