This video has some good advice on bra fitting and what to look for when walking into a store.

Opposite to others, it doesn't tell you to add 5, 4, 3, 6 inches to your ribcage, just look for the number you just measured.

When you go into the sotre to try on different cups, find the band size with the cup you think you are. Try that on. If it doesn't fit in the cup, try another size and keep trying until you find one that you think it fits you best.

Or, get professionally fitted .

I also liked the observation she makes about stores that don't carry a wide variety of sizes and their fitting skills.