My daughter wanted me to laser engrave something on her jeans. So I wouldn't upset her, god forbid I ruined them, I firsted tested laser engraving denim with an old pair of my Levis or Lees, maybe they're Wranglers.
It seemed to work pretty well, I had to reduce the laser power several times so I wouldn't damage the denim material to much. Eventually I figured out a good combination of speed and power and dpi.
Anyway - see the pics below, the settings I eventually wrote down as my future reference are:
The Oriental Dragon on the blue jeans was for my son.