Think of shopping for bras like shopping for jeans: some styles fit and flatter more than others. Similarly, some bras will suit your natural breast shape more than others. If you love the idea of a perfect coverage style but the cups don't hug your curves, try a demi or push-up. Finding the perfect bra is an art, not a science.
Knowing your size is just a starting point to finding a bra that fits well. Most women stick to always buying the same size -- and end up with too many bras that aren't right for them.
Actually, many of your bras should be different sizes, because the fit is so much more important than the number on the tag, and varies among different brands and styles.
The more bras you try on, the quicker you will be able to recognize when a bra fits well -- and how it should look when it's most flattering to your body shape. Start with the size closest to your measurements, and then try on more. It also helps if you try on a few different styles of tops with your new bra. If you ordered online and you're trying on bras at home, this will be easy. If you are out shopping, wear a form-fitting T-shirt with a button-down blouse on top -- and try on the bra with each separately. With a little practice, you'll become your own bra fit expert -- and you'll have a lingerie drawer filled with more comfortable bras.