SIZE CHARTS and a link to some guidelines
EVERY brand says that their Small is for younger users who have not had children that and their Large is for those over a certain age and/or have given birth. But what they are called doesn't fall within any range of physical sizes. For example, some "smalls" are larger than some "larges." Don't pick a brand or a size first. The chart I've linked to is divided into three widths and 6 lengths. You know your body best. Figure out where you fall within those length and width categories and then choose from those cups that fall within your fitment parameters.
Due to the layout, the chart didn't fit without getting cut off any more so go to
http://sizecharts.livejournal.com/
for (hopefully) a better version of it.
And here is a link to ANSWERS TO COMMON QUESTIONS: http://kuradi8.livejournal.com/