For bras in general, I've read a lot lately how women are getting measured wrong in stores or are using online tools that are NOT giving acurate results. Because I'm nursing right now, I know how especially sucky it is to be fitted properly when you have constantly changing putty boobs. So, since I don't have another bra review for you yet, I'm listing just *some* (there are hundreds) sources for helping you determine your bra size. I tried Leading Lady and Herroom and they were pretty accurate.
Leading Lady - Find Your Fit
Women's Health Magazine - How to Measure Bra Size
Wikihow - How to Measure Your Bra Size
Bare Necessities - Find Your Bra Size