Below is a breakdown of how I use layers to animate in Flash. It's helpful to animate this way when I have some parts that stay still while others move.
1. The Final Clean up Animation (which has yet to be colored)
2. The Body Layer which is held for a long time.
3. The Arm and Boob Layer which moves unlike the body layer below.
4. The Head Layer, which moves, but not as much as the arms and slightly more than the body.
5. The Mouth Layer, for lip sync and so I don't have to redraw the held (still) parts of the rest of the head.
Hope it was educational!