Most women wear their bras too big—they think "comfortable" means "loose." What they actually need is a firmer band to keep the bra level across the front and back.
Cup size doesn't remain constant—it increases and decreases proportionally with band size—so if you go to a smaller band you may need a bigger cup. (Example: You think you're a 36B, but your bra isn't fully supportive? Consider a 34C or D.)
The average woman's bra size changes six times during her life—weight loss or gain, pregnancy, and age are all factors.
Most of us have breasts that don't match perfectly. If that's true for you, take your bras to be altered by a tailor.
Buy three top-quality bras rather than a lot of cheapies. You need only one to wear, one to wash, and one in the drawer.
Put bras—even fancy, lacy ones—in a net bag, use mild soap, and run on the delicate cycle.