A few tips for picking out the right bra - Trouble shooting fitting tips
Is the back of your bra riding higher than the front?

This usually occurs when the band is too loose (which sometimes can indicate a worn-out bra) or the straps are too tightly adjusted pulling the back upward.

Check strap adjustment and measure torso to determine if a smaller band is needed. Also, if the sides and the back of a bra are made of long stretch elastic, it can sometimes cause the band to ride-up. If so, a bra of a more rigid fabric might be preferable.