Good Housekeeping has a brief bra fitting guide here: http://www.goodhousekeeping.com/beauty/fashion/best-bra-guide-0800
Taking Your Measure
Not only will a properly fitted bra feel more comfortable, but it will make your clothes fit and look better. First step: Learn how to do the math, so you can figure out your own dimensions.
1. Stand straight and relax. Don't inhale and expand your rib cage as if you're about to blow out candles.
2. Using a soft tape measure, measure all the way around your body, placing the tape measure right beneath your breasts (example: 30 1/2). Add five to that number, rounding up if necessary to the next even number (36). Congratulations: That's your band size.
3. Next step: The cup. While wearing a bra, measure completely around the fullest part of your bust (across the nipple) without pulling tight. Make sure the tape measure goes around your entire back (38).
4. Subtract the number from your band size (38-36=2). The result will indicate your appropriate cup size, based on the following guide:
0=AA 1=A 2=B 3=C 4=D 5=DD 6=DDD