i wanna share this tip with you. actually, this is not my idea. that was nice guy Roma who had shown this settings to me Main thing here is turned off "spacing". it works with cs5 and cs6 (and i don't know how about other versions).
brush, used here is STANDARD soft brush.
i use this smudge brush in other ways too - to make borders softer, to blur and so on. Use various pen pressure, stroke speed and stroke length.

*updates:
icekatze said: I can confirm that this works with Photoshop 7.0 (circa 2002) so I suspect that it will work with any of the CS Photoshop programs. (The only difference is that the "transfer" tab is called the "other dynamics" tab in 7.0, but it does the same thing.)

http://kir-tat.deviantart.com/art/motion-blur-with-smudge-tool-442918443