The idea of a GoBo is one that many experienced photographers are aware of, but many inexperienced strobists may not know. Basically, you use a GoBo to go in between a light source and the subject in order to modify the look of the light. The folks at Good Light! magazine have a nifty trick for you involving the use of a large white shopping bag. Of course, it needs to be semi-transparent, but basically it turns into a softbox for a flash that can be used off-camera and diffuse a subject with very soft lighting because of how large it is.
They show it off by shooting the upper half of a subject–which makes sense due to the size of the bag. The photographer is able to effectively produce soft light onto the subject and give a great look by also killing the ambient lighting.
The video is after the jump give it a try and use a bag as a flash modifier.