I second what David said. I just shot a short film with multiple flashbacks and all we did is pan the camera away, defocus the lens and rack the iris a bit... My theory is that a film's audience is usually passive and if you feed them a visual language, they are likely to understand it. We'll see how it comes together. I also like the flashbacks in "the limey." They shot them the same way as rest of film, just messed with the shutter for effect.