I used to be a bra fitter... and I can tell you I have seen it ALL. ALLLLLLLLL...
And I used to do EVERYTHING in my power to fit a woman in the RIGHT bra for HER. And I could do that 99% of the time. (Honestly, the toughest to fit were women who had augmentations -- european bras tend to be better for augmented breasts, btw).
A my Top-10(+1) tips that might be helpful for everyone.
#1 most important thing... ALWAYS... is get your band size right. This is the MOST important thing about your bra. It's what does the supporting -- NOT the cup. You can measure yourself at home if it's more comfortable for you. Fold over the measuring tape at 5 inches and then measure. Pull the tape snugly, but not TIGHT just below your breasts. THAT is your band measurement. Cup size is secondary and fairly arbitrary depending on the brand. The band should NEVER cut into you. and it should NOT ride up your back (too big of a band size). It should also fit on the middle clasp...
#2 -- DON'T be afraid of underwire! If a bra fits you correctly, it will not dig into you. The underwire should come up toward the middle of your armpit. (amazingly, if you see the new VS ad for their new miracle bra, you'll notice the bra does not FIT her AT ALL because you can see how the underwire on the side comes up about two inches away from her airmpit, and she's spilling over the cup)
#3 -- Don't be afraid of a little "lining." If you're larger, it WILL NOT make you appear larger... but it does help shape and lift a little and is good for "modesty." Lined is not the same as PADDING.
#4 -- try on SEVERAL different types of bras. Try a full coverage, a demi-cup, lacy (except when you're wearing t-shirts!), not lacy, lined, unlined (though, please refer to #3). Try something new and don't get stuck in the mindset of wearing a bra style you're most familiar with... You could surprise yourself.
#5 -- You should NEVER have quad boobs. EVER. If you're large -- find a more full-coverage cup that smooths you out. Your boobs should not be split in half by a too-small cup. EVER.
#6 -- Once you get your band size right, go ahead and try on bras with the same band size and different cup sizes. It varies -- stupid bra manufacturers.
#7 -- Another reason bra manufacturers are stupid is that you can actually go down a band-size and up a cup size and the bra will generally fit the same. (For example, a 38B fits almost exactly as a 36C... a 38C same as a 36D, etc...) So if you're having problems finding the right size, go ahead and try this trick... but I would only suggest doing it for ONE SIZE difference..
#8 -- Minimizer bras can work wonders for very large breasts. Just be sure you can breathe in them!
#9 -- Don't freak out over brands. If a bra fits well, it doesn't matter. (honestly, the selection at places like Kohls is INCREDIBLE)
#10 -- I know it's been mentioned before but Playtex makes "Nearly" bras to fit all of you who are "in between" sizes.
#11 -- If you have one breast larger than the other, don't be too ashamed to wear "chicken cutlet" inserts to help balance you out. Or find a bra with removable padding -- you can keep the padding in one and take it out of the other. Don't be embarrassed about the "cutlet" no one will see it -- and anyone who would see it has seen your breasts anyway...
The right bra will make all the difference in the world... It will make you appear thiner, make you stand up more straight and you won't be chomping at the bit to get home to take off your bra at the end of the day.
If any of you have questions, PLEASE don't hesitate to PM me. I went through a ton of training, and AGAIN have seen it ALLLLLL. NOTHING surprises me. If you have a problem, I PROBABLY have a solution or some suggestions.
Good luck ladies... we're all in it together...

but I have faith that there is a bra for EVERYONE out there.