Rendering as still sequences using .png (which preserves transparency) can help you speed up renders, if you have a video editor capable of using layers, you can use stills in the background as recomenended by Skuts another user here. Link to one his excellent vids here

http://www.youtube.com/watch?v=f4eJNbxLjEk

Rendering in layers is also the best solution for rendering multiple figures that I've found, other than lo-res figures