In order to correctly wear your bra, you must understand the basic features of your anatomy. Your bra should be aligned with your breastbone and the underwire should not be pulled down so as to bring to bear full functionality of the support. This is important in particular after breast augmentation when you do not at first realize that support can be beneficial to you. Even though your post-operative results are to a large extent self-supporting, this support must still be increased by a firm bra in order to make it last.
Relaxed underwire may be caused by either laxity in the shoulder straps or the size of the band. On the other hand, if you tighten the straps too much, the back support will slide up your back. This may happen because the bra is too large and in trying to make it fit you are basically compensating for the front fit by the back misfit. If you feel the back band sliding up, you need a smaller size bra. Now, smaller size does not necessarily mean a smaller cup. Post-augmentation, it is preferable to have a slightly larger cup (up to +2 sizes) until you mentally, psychologically and physically adjust to your new size breasts. Many women injudiciously trade-in only to find out that what they needed was a soft-cup, larger-size temporary bra.