Photo by the incredibly talented Beat on Flickr
Some of the telltale signs you're wearing the wrong size include the ladies falling out of the cups when you lean over, the dreaded "four-boob," and generally spilling on the sides, bottom, or top of the bra. It's quite possible that as insane as it sounds, if your bras are not fitting you could be bigger than a D cup. Bigger than a DD even. Do they exist, you ask!? But of course, and not just by custom order or specialty store!
I recently discovered, through a marathon dressing-room session at Marshall's-- wherein I snagged all of the biggest bras I could find in several different cup and band size--, that I am not, in fact, a 36DD as I had assumed all of these years, but more accurately a 34FF. I allowed myself a moment of eyes-popping-out-of-skull before remarking how amazing I felt when properly supported and forgetting all about that pesky, "embarrassing" size. Forget about the number (or letter(s) ), once again, ladies. And don't make the mistake of just going up and up in band size if you don't feel your bras are fitting properly; most of the support comes from the band so if it's too loose you're doing yourself no favors! A good band will actually eliminate the back fat situation (if you've got lots popping out above and below, try a wider (3-hook minimum) band instead of a bigger inches measurement.
If you are unable to find a good fit on your own through trial and error, do make a trip to a lingerie store (not Vicky's or Frederick's; one that specializes in foundation garments, not bedroom outfits!) such as Lady Grace, and be professionally measured. Once armed with your new knowledge about what size actually fits you properly, GO SHOPPING! Lady Grace and similar specialty bra shops have plenty of stock but it may be heavily weighted toward the hideous category. Big boobs can have pretty bras, too! Fayreform is a new brand catering toward full-figured ladies and their high-end options can be found for as little as $7 at Marshall's, so keep an eye out. They are also carried at better department stores like Nordstrom, where you might be able to find a seasonal deal to bring them down from the $60 range, and HerRoom.com sometimes has them on sale also.
A few other excellent web resources:
HipsandCurves, catering toward plus sizes (with band measurements beginning at 38 and cup sizes up to J)
Bravissimo for the larger cup sizes (D-KK) with smaller band measurements (28-40)
BiggerBras carries every size imaginable, pretty much, although the interface isn't perfect and the sales aren't that inexpensive
BraExperience has a huge variety of sizes and styles, and a decent sale section, as well as a promo running right now! (use promo code "BE55OFF" when adding to the shopping cart to receive 55% of Sale Priced bras. (Only Valid on Style # beginning with dsx))
I've also heard wonderful things about Lane Bryant's boob-wrangling devices, though I have no personal experience.
Have you been fit properly for a bra? If not, do you suspect you could be wearing the wrong size? I hope this has been helpful. Get out there and treat your girls right!
Happy bra shopping!