Bra Band
The band of the bra should be straight all the way around. If the band is riding up in the back you need a smaller band size or to tighten up the band. Your band should feel firm and secure, but never binding. You should be able to slip two fingers in beneath the band in the back, and one in the center in the front.
Straps
Most of the support comes from the band of the bra not the straps. Bra straps shouldn't dig or put strain on the shoulders. Consider a smaller band size if your straps are doing most of the support.
Cups
If you are spilling out of the cups try a larger cup size or a style with a fuller cup. If you do not fill out the cup or the fabric is wrinkled you need a smaller cup size.
Center panel
The center panel should lay flat against the chest wall. If it does not you need to go up a cup size.
Underwire
The underwire of the bra should encircle the breast. It should never dig or poke into the breast, if it does you should go up a cup size.