Fitting Your Bra
Everyday bras
To start, it is good to have your measurements. You can usually have them professionally taken for a small fee at most lingerie stores, and some department stores. It is a good starting place. Try on many different styles to determine what you feel is most comfortable and fits your personality. Things like underwires add support to the bra, and padding (usually foam) adds inches and contours. Think about what kinds of clothes you will wear with your bra in order to avoid discomfort.
As you try them on, bend at the waist and pull the bra away from your breast using the straps. This will help you determine whether or not the breasts fall naturally into the cups. The bands and the strap should be snug, but they should not dig into your skin. And buy a bra that feel comfortable on the loosest hook and with the straps lengthened. The bra naturally stretches over time, so it is nice to be able to tighten it up.
Jump up and down while trying the bra, and swing your arms. Walk around in the dressing room a bit. All of these are things you will do while wearing the bra, and you wan to be sure that you will be comfortable.
Sports bras
Find a size that is close to your proper size and then try that bra. While many sports bras are pulled on over the head, there are some that fasten in the front. Make sure that the straps do not dig into your skin, and check for general comfort. Large breasted women need to make sure that pressure is not added to the shoulders for good support.
Take into consideration the type of material that the bra is made from. Things like 100% cotton, while supportive, can be uncomfortable because it traps sweat moisture next to the skin. A cotton/polyester/lycra blend will wick the sweat away, but support your breasts.
As with trying on a regular bra, moving around to ensure proper fit during activity is a good way to make sure that the bra is a good fit.