OK, I can't resist popping in here because I'm still wondering why it took me six decades to figure out what kind of bra I ought to be wearing.
If I'm any example, I'd have to say that most of us don't have a clue as to what we ought to be looking for in a bra other than rudimentary band and cup size. And, in my case, my ignorance was compounded by a big dose of attitude that only "certain types" of women (porn stars? mistresses? rich, thin second wives?) cared about their underwear. It took a careful fitting, a trainee SA who had to explain what she was doing to her supervisor as she fit me (with me listening closely!), and some internet research to finally realize that PPP ought to apply to my bras as well as my jeans.
I think that every woman, in addition to knowing her starting point with band and cup size, ought to know (1) her own breast shape and placement , (2) what bra styles will be most flattering and supportive for her shape, and (3) what brands are most likely to work for her specific needs. At a minimum, I think every woman ought to know how her breasts are spaced on her body, whether they are full on top or the bottom, and what kind of shaping she expects from her bra.
A bit of knowledge about bra construction also helps narrow the field. Different styles and brands use different types of underwires--some are thicker than others, some are U shaped, some are broader with more of a "smile" shape, some are longer so they hit higher under the arm, some are low and shallow--and finding the right shape is key if you want to be comfortable. Gores (that little triangle piece between your breasts) also come in different shapes and sizes. The same goes for strap placement--some straps start near the centre of the cup while others are located near the arm. Cups also are shaped differently depending on the brand and the style--some are designed with a larger bottom piece and a narrow top, some have a full top piece and a narrow bottom section, while others use a third piece at the sides to help centre widely spaced breasts. Some women find a narrow, one- or two- band comfortable, while others prefer a three- or four-hook style. Some brands use stretchier fabrics than others which some women find more comfortable while others decry the lack of support.
I'd also suggest that learning a bit about your own shape, and what styles and brands might work, is a far better way to work with a bra fitter to make sure you get what you want. After working with other bra fitters, I came to realize how lucky I was to get that trainee in my first fitting. Bra-fitting is part science and part art. Fitters see a lot of clients in ill-fitting bras so they sometimes develop a "firm" approach in telling you what you ought to buy. To my mind, collaboration is a much better route. If a fitter doesn't want to listen, or can't explain why she is recommending something, I just thank her and move on.
Some sites that I found useful in figuring out my own shape are
http://undiegamer.com/bra-fitting-advice/
http://brasihate.blogspot.ca/2.....op-vs.html
I also found that http://www.bratabase.com/ really helped me understand the enormous difference between difference styles and brands.
Finally, I just want to say that if I had a daughter, I'd make sure that she learned about how to figure out this whole bra buying business. Bra buying ought to be as much fun as buying shoes--and I think if we, as women, started thinking of it in the same way we do all the other items we put on our bodies, companies and stores would soon make sure that buying our undies was as pleasurable as buying a cute pair of shoes.
ETA: I just can't resist adding the "decorative" bras can be "everyday" bras. Cheap lace can be scratchy, but some of my softest, most comfortable bras have lace on them. As for the invisibility factor, I actually find some of my lace bras much less visible under thin knits than a bulky molded bra. The same goes for nipple show--well-placed seams can render them invisible, but, if I want extra insurance, I use one of those small silicone patches. Below are a couple of my lace Empreinte "everyday", T-shirt bras (and they come in colors as well!):