Saufnase's Tutorial on creating a 10 second shadow
All the pictures POP to full size.

Alright, lets get started.
Have a background and a nicely cut out layer of whatever you want to have the shadow.
Duplicate the Layer and give it it's proper name. Make sure the NEW shadow layer is underneth the original layer.

For the new shadow layer, adjust in the Hue/Saturation tool (Ctrl-U) the lightness to -100.
Now, use the Free Transform tool (Ctrl-t) to adjust the shadows directon. Holding down Ctrl while moving any points helps a lot for this operation.
Some really funny Gaussian Blur for diffusion.
There, almost done.

Now adjust the opacity of the layer to the desired level.

Done.