The elastic tends to stretch with wash and wear use so that column thingy on the back may need to be tightened. A good rule of thumb is to anticipate this stretching and buy a bra that fits well on the looses column to give you adjustment capability.
If you notice bulging under or near your arm pit area that's a sign that you may need a larger band width.
If you recently gained weight, or may need both a larger cup size and band width.
If you noticed a " puddling/ over flowing " over the top of your bra , you may need a larger cup.
Trying on new bras and looking in the mirror usually lets me know if its time for a bra size change. ( hint: After trying on a new bra, put a t-shirt over it and take a good look in a mirror to see if it fits well )
You can go to a specialty bra/lingerie store and get a sales clerk to measure you and help you find your perfect size .