Bitch PhD explains it all in a DDD-sized post on bras. How to measure your proper bra size (the odds are excellent that you should really be wearing a smaller band size and a larger cup size). Where to shop. What brands to seek out. How to check for proper fit. Maternity/nursing bras. In the comments section, there are many more reader tips for situations Dr. B doesn't address, like bras for small-breasted women. Go read it, ladies, and send the link along to the women you know.
My only quibble is that Dr. B says to use the tightest set of hooks on a bra, whereas Oprah's bra experts said to use the loosest (leaving leeway to tighten the fit as the bra stretches out over time). I vote for Oprah's method, as it lets me fit comfortably into a smaller band size than I'd been wearing.
Additional info: Click this link for more on determining your correct bra size, courtesy of a bra-fitting expert who contacted Dr. B.