With the permission of the author and RetouchPRO member, Shan Canfield (SHAN)
, attached at the BOTTOM of this post is a tutorial she wrote that details a very creative variation to the well-traveled and frequently described "duplicate background, desaturate it, duplicate the desaturated layer, invert, set blend mode to color dodge and apply Gaussian blur" sketch method.
This tutorial is among several at Shan's site that I found to be especially well written and informative (scroll down to 'Pencil Sketch'): http://www.shanzcan.com/photoshop/photoshop.html
While you're at it check out the rest of the site as well: http://www.shanzcan.com. SO WHAT'S NEW HERE?
Note steps 7-12 for some new ideas that take this method to the another level.
But don't stop there. In step 7 try other filters or filter combinations on that color dodge layer as well. You will probably get variations from the "standard results" were quite compelling. Results from a heavy dose of "Dust and Scratches" and a combination of "Sprayed Strokes + Ocean Ripple + Colored Pencil" were quite compelling (to me, at least).
A special thanks to Shan for permitting me to attach the Pencil Sketch .pdf in this thread as well as a much-needed dose of creative inspiration.