Well first you need some breast MTs for movement which can be found at great sites like Morph World (link on the side bar). Next, animators usually adjust these themselves knowing when they will bounce and such. Nerd has a tutorial where you can automatically tie a MT to a body part rotation so, as you lift the arms, the breasts could go up. For the most part the animator adjust these themselves. Alsom make sure you're looking at a poser animation. Animation master can do this too but it uses patchs and splines with control points.