Can't find a bra that fits? You're not alone. The statistic that floats around is that more than half of all women wear the wrong sized bra.
Yes, it's a hassle hunting down your real size, and a well-fitting bra, but the difference is worth the effort. You won't spend all day hitching your straps only your shoulders, wriggling as wires poke you and stab you, or whining about red marks in your flesh when you get undressed. The perfect bra can be more comfortable than going braless.
OK, the very first thing to do is go and get measured. And get measured by a specialist. Go to the smartest department store in your town, and get them to whip out the tape measure. The fancier the shop, the more likely the staff will have been trained to be polite and discreet. It's more embarassing to have a lumpy chest spilling out of a badly fitting bra than it is to be semi-naked with an uninterested stranger for a few minutes. Prepare to be surprised by the results.
(Marks and Spencers, in the UK, offers a fitting service, but it's a waste of time. Their training is so poor that they can't really help you out here. Try Selfridges instead. An example: sick to death of finding bras that were just all wrong, having been told one size by M&S and sticking to it for ages, I finally got measured properly. There are so many arguments about how to measure back size for a bra, that it's worth just going to someone who knows, and then you don't have to worry about adding 3, 4, 5, even 6 inches to the measurement you get, rounding down, standing on your head, and crossing your fingers. I stopped wearing a 38C and started wearing a 30D/E, and all was comfy and happy. No, that's not a typo.)
Secondly, you need to know what a well-fitting bra actually feels like. The main back strap should fit. It shouldn't be loose, or ride up at all. If it arches up between your shoulder blades it's too damn big for you. Get someone, ideally the changing room lady, to try giving the strap a tug. It should only have a couple of finger's space when pulled, maximum, and should pull you backwards rather then stretch into space. If you are not super-slender-skinny-slim, well, you may have a little bit of flesh bulging from a strap. Don't worry about that. As long as it's not cutting into you and leaving marks, it's better than a bra that's too big. The underside of the front should sit tight and snug. You should not be able to see an air gap between fabric and skin. The part between your breasts is where you can get the best idea of a bra not fitting. If the bra is underwired double check that the inside curves fit neatly against your body, and don't gape. If it's the right size, underwires are just fine and dandy. Wrong size, and you are risking damage to your body.
The cup fits if it looks and feels right. Obvious, but look very very closely all around. It shouldn't be pushing any of your flesh out of the bra and into your armpits. No wires should be poking into you. You shouldn't have the dreaded four-tit-syndrome, with the depression of flesh, and then the spillage of extra boob out of the top. You shouldn't be swinging around loose inside the fabric. Get the assistant to help you out here. A second opinion is always helpful, and the assistant is unlikely to be slavering all over you hoping to unfasten it as soon as possible (lovers are very good at judging the effect of underwear, but not of the perfect fit).
Thirdly, know about how sizing works. Most bras are scaled up from an A cup. This is why cup sizes get so weird when you hit D and above: they get so much bigger with each size, and often come way too far up the breast. The difference between a C and a D is far more extreme than between a B and a C. Some manufacturers are now producing larger cup sizes that are based on a D or a DD, which will fit the more generous of breast better.
You can sort of cheat the sizes, too. If you are, say, a 36C, you'd get much the same effect from a 34D or a 38B. They wouldn't fit, exactly, but it's a good way to see how the scaling works and how the amount of fabric in the different cup sizes varies. This is why going from 38C to 30D/E makes sense. down 4 back sizes, up two to four cup sizes...of course, no one makes a 30E, so I had to wear a 32. No, I'm not this shape anymore. Yes, this stuff is complicated.
Fourthly, be prepared to buy different sizes from different manufacturers. Not all bras are cut the same. Not all breasts are made the same. Try on dozens of different bras: the shapes are so different, and not all of them will fit (even if it's officially your size). The sensible special in cotton from Company X might be all cover-up and high cut, but the friday night lacy balcony number from Company Y, might tip you out and leave you wobbly if it's the same size.
Fifthly, be honest about your shape. Large breasts are heavy, and can't be held up by the little wisps of lace and hope that smaller girls can run around in. Or, if you are economical of breast, don't go for padding beyond about a single cup size up: you'll look unbalanced and feel uncomfortable, and give someone the surprise of their life te first time you get naked and shrink.
And don't forget to go shopping again if you gain or lose weight...