Attached Link: http://www.wacom.com/tips/tip.cfm?ID=10&category=Photoshop

Boni's tutorial reminded me of a nice one I'd found at Wacom a while ago.

This one uses Photoshop's Find Edges command and an adjustment curve to automate the process. Results obviously aren't going to be as precise as hand drawn, but there's nothing to stop a person from going in and cleaning the automated results by hand. Results will depend on the amount of contrast in the original image. It's well worth trying this technique to see if it's quick results are worth pursuing further on a particular image.

Another nice effect is to create a duplicate layer of the image and apply this effect to the layer and change the layer to multiply so that the lines show on the photo.