CTRL + ALT + SHIFT held down when you are in the Free Transform mode will give you the PERSPECTIVE tool! Just hover your mouse near a center control box and you’ll see an extra double arrow appear next to your cursor arrow. This will allow you to stretch the shadow so it looks like a long shadow cast by a low angle light.
In the LO below, you can see the results of a Perspective Transformation if you look closely at the shadows on the “Tickle Monsters” at the bottom.
(Click the image to see a larger version)