Like a previous poster said, ditch the underwire bras for clinicals! At my school, all the ladies wear sports bras to help each other out when practicing auscultating heart sounds. See if your partner or whoever you are practicing on would be willing to wear a sports bra while at clinicals so it will be easier for you to maneuver (or for her to maneuver) her large breasts out of the way. Modesty is of course important, so I would first ask her to move her breast out of the way or let her know that you will be moving her breast as to make it easier to find the apex/PMI. Don't act nervous and it will put the person more at ease. Like "you do this all the time" kinda thing.