Bra Fitting

Yes, it's true that most women are wearing the wrong size. However, the standard advice on how to measure for fit (the old 'measure under the bust and add five inches' formula) is simply incorrect for most brands. You can get close to the correct cup size by measuring the difference between the under-bust measurement and the fullest-bust measurement, but even that is not an exact formula -- your best bet is still a professional fitting. If you're own your own, the best rule of thumb to follow is to make sure that the cups don't have any overspill (at the top or the sides) and that the band is at the same level all around your torso - if it's going up or down, it's the wrong size.

General Guidelines
Cup volumes vary depending on the band size -- this seems an obvious point if you have tried on too many bras, but it comes as a surprise to some. The cups of a 34C and a 32D bra are the same in volume (if their manufacturer is true to size, of course); likewise, a 36B and a 34C are also size-mates. The difference is in the band; that's where the bra is doing all the support work (and not in the straps as some assume). Quite often, women are wearing a band size that is too big -- this can be hard to pin down since a band that is too large rides up in back which throws off the fit overall. In the US in particular, it is harder and harder to find smaller band sizes in stores -- a 34 has become the standard 'small' end of the scale, even though it's actually very much in the middle.

Styles
Fit can vary enormously among various styles -- even for the identical size. A full-coverage bra and a demi bra can be really quite dissimilar; a sports bra is entirely different again (and within sports bras there are a number of sub-categories of the style). You may have found your size, but not all styles will necessarily be a good match for you (or for a particular outfit). The Bra Geek has never found a t-shirt bra that works for her, while many women swear by them.

Frequency
It might seem like the industry suggestion that you have a fitting every six months is a ploy to drum up business; while that may well be an element, even the best bras start to wear out after a few months of use (and washing - even hand washing). While all the parts may seem to work, eventually the support simply goes. And of course all the usual size changing elements - new baby, new workout, new medication, etc. can all have a major impact on your size.

Good advice
Sensible fitting advice can be found on Rigby & Peller's website; Figleaves and Bravissimo also have sound fitting advice. While the online bra size calculators are fun, they are almost never accurate. That's not to say don't order online (heaven forfend!), simply that you should know your size (and favorite styles) before you do so.