Well, not the best technique, I am sure, but one I have used to good effect:
Press an x-acto blade into the area where you would like the scar. As the blade sinks in to the pewter, it rises on either side, creating an effect of "bulging flesh" that appears along the edges of a laceration.
Carefull when doing this, though. There's a very real possibility of comparing the miniature to the cut you just gave yourself...
Ghidorah