Problem Solution
The band digs in (it feels too tight) Try a bra style with a wider band or increase the band size (i.e. 10 to 12)
The bust line sags (your boobs are drooping) Try a style with more support or avoid bras made of stretchy fabrics or shorten the straps
The cups overflow (your boobs don’t fit in) Try increasing the cup size or try increasing the band size or try increasing both cup and band size: 
The straps dig into the shoulders Choose a style with wider or padded straps or try a style with longer straps, try decreasing the bra/band size (breast support should come from the rib cage band, not the shoulders)
The cups are under-filled (cup material wrinkles at top and sides) Try increasing the cup size
The back of the bra rides up Try a smaller bra/band size or adjust the backstrap fastener tighter or try lengthening the shoulder straps
Underwires dig in under the arms or stick out at the front Try increasing the cup size
Black bras are tighter in the cups. Why? If you have the same size and style bra in a few colours and they all fit the same except the black bra, this is because the dye in the black bras effects the material and may make the cups or band tighter. When buying bras, ensure you fit all of them comfortably before paying for them as you may find that they are not what you think they should be