Most Helpful Guy
Breasts do shift depending upon what posture you're holding at the time. If you are in the United States and live near a major shopping mall, go and pay Nordstrom's a visit. A former girlfriend of mine used to work there and they have the best bra fittings around. Those ladies know their stuff! Many women often purchase bras that are the incorrect size or shape for their bust.
As a younger girl, get the advice of those ladies who have been at it longer than you have. You can't go wrong with Nordstrom, I've sent several female friends who have complained there too and they've always been happy with the results. If you have no Nordstrom nearby, The Gap does them as well, but generally speaking their staff isn't quite as knowledgeable.