Wearing the proper fitting bra will improve your posture, appear to make you look thinner and your clothes will look and fit better.
A bra’s main support comes from 90% of the band and ONLY 10% of the straps.
A new bra should be fastened on the loosest (first ) hook and should be the same level all the way around.
The bust should be midway between the top of the shoulder and the bend of the elbow.
Your bra band should feel tight and firm with enough space to insert two fingers under the back band.
The whole breast should be contained in each cup with no bulging or spillage on the top or the sides and the cups should not pucker or wrinkle.
If the bra is an under wire, the bridge or center gore should lie flat against the chest wall.
Wash your bra by hand or in a longerie bag in a mild detergent. Cool water rinse will refresh a bra by shocking the elastic stretch memory to return after wear and NEVER put it in the dryer.
Don’t wear your bra two days in a row. The elastic needs a day to relax and bounce back.
Have a  professional bra fitting once a year or if you have change in weight, level of activity or pregnancy.