First off, let's identify what breasts are, for the most part : fat. Soft, delicious fat lying on the ribcage. For the purpose of this physics demo we're going to be dealing with larger, fattier breasts, because they'll hang and deform in ways smaller breasts won't. Well, they will, but in a less noticeable (and therefore harder to draw) way.
Unless bound by clothing or filled with something other than natural fat, breasts will react to the effects of gravity, being pulled down in a pendulum fashion.
Breasts are attached to the pectoral muscles, so when the muscles move, they move. You can think of it like a marionette puppet. Pull one string, and one moves. Pull two, and both move.
Breasts tend to flatten out / deform when they come into contact with surfaces, even themselves.
Breasts move as a unit. If blocked into a box, you can split it down the middle to help determine perspective. As the box squashes, rotates and/or skews, so do its contents. Also included in this image is an example of how breasts can stretch and squash if manipulated by outside forces. They are amazingly malleable!
Breasts are affected by motion, somewhat less so when bound by clothing. They will take longer to "catch up" to whatever the action is, so will drag behind, seeming sometimes to swing in the opposite direction of the motion. This, along with the effects of gravity and deformity, contributes to a look of weightiness.
There's no end to what breasts can look like and what they can do! Hopefully you'll be able to "plan" them more carefully by keeping some of the above in mind.
