Hi, I found a cool way to make a "Clean Background" from your shot even if you didn't take any. Here is how it works:
First, your target object ?the one you want to rotoscope? MUST be moving in deferent places in the shot. Also the camera angle should be fixed. Then, take a snap shot of the first frame, next, play the clip until the object "or character" move to another area in the scene "i.e. the previous area covered by the object in the first frame is now showing" and take another snap shot. Now, you are the genius, use both stills to get a clean background "without the moving object on it".
This is specially helps those who they have only one shoot and they want clean background to do rotoscope, but they can't go to the same place again to shoot from the same angle.
The source of this trick is located in: http://www.livedv.com/magazine/2000/...ledge0400.html

There is a lot of shooting tips too.
Have fun.