Flashbacks

Flashbacks are a trademark of SAW movies - in these movies to show that these are flashbacks they are in a slight blue tint, which also makes it look quite cold.

Ideas of how to make the flashback obvious to the audience:

Blurring the scene

Showing the scene in Monochrome

Adding a misty border around the edge

Applying echoes to any digetic sounds

Showing the scene with a grainy effect

Showing the scene in Black and white

Showing the scene with a slight tint of some sort of colour

Films that use flashbacks and how they display the flashbacks:

Wayne's World - uses idiosyncratic wipes (when a film uses strange sequences to move between scenes)

Saws - Different coloured tints with a heavy use of light and dark. (flashbacks were usually darker and harder to see than the present time)

Big Fish - All of the flashbacks include brightly coloured objects and have a slightly glowing effect on characters faces.

Boggy Creek two the legend continues - every flashback slightly blurred.

G.O.R.A - grainy and black and white film.

Trick R Treat - Flashback filmed with a brighter colour pallet than the rest of the film and bathed in golden light.