This expression maintains the objects apparent size as it moves away from camera.
It's helpful for taking a flat layout and making a "multiplane" layout from it for animation.

Apply this expression to your 'Scale' attribute and it'll automatically 'flip' the layer in 3d space when the camera goes behind it. This way, your text will never be seen from behind, for instance.

Apply this expression to the opacity attribute and it'll disappear whenever it's flipped away from the camera. You need to have an active camera in your composition for this one to work.

From Adobe's LiveDocs

In this video tutorial, Creative Cow Leader Aharon Rabinowitz shows you how to simulate airflow over an object by using the After Effects CS3 puppet tools with a particle system.

A Macro that allows you to set objects at Z distances and creates a camera that pans, dollys, cranes etc keeping parallax. Also calculates the relative focus for each layer including using hyperfocal which can be turned on or off.