To find the perfect fit in a bra, it is first important to understand the basic function of a bra. Anything with straps suggests a support via hanging component. This is actually incorrect when it comes to bras. A bra should provide roughly 80 percent of the support from the band circling around the bottom of the bra. Put another way, a bra should be doing more pushing up than hanging when it comes to supporting the breasts.
Overall, a bra that fits perfectly will be comfortable first and foremost. When wearing it, your breasts should be sitting midway between your shoulders and your elbows. The band should be snug around your upper chest and back, but not too constricting. No part of the bra should be leaving red lines or spots.
The band running around the base of the bra is incredibly important. This is where you are going to get the vast majority of your support. Get the band right and you will feel much more comfortable. When buying a bra, start with size that fits on the loosest hook. Now constrict the size so that the band is firmly fit around the body, but not too tight. Your goal is to have the band run parallel to the floor around your body. It should be as high on your chest as it is on your back. A quick way to test the fit is to see if two fingers can fit snugly under the band.
The cup size is our next most important factor. Be honest with your cup sizes. If you are bulging out of the cup, you need to go up a size. If there are gaps between the cups and the breast, you need to go down a size. It is that simple. With underwire bras, you can adjust up or down to make sure the wires don't press into the inside of your breast or arm.
Bra straps are incredibly misunderstood. They are supposed to be supplemental support to your girls, not the primary support. The band serves that purpose. To get the perfect fit, your bra straps should be secure but not tight. If they leave red marks, they are too tight. It is better to have them to loose than too tight, so adjust accordingly.
That is it. Focus on the general concept of support from the bottom up and you should be able to find a bra that is perfect for you.