Unfortunately, to do a gravitational slingshot, you need a body other than the one relative to which you want to accelerate. So you can use Mars to accelerate r

Unfortunately, to do a gravitational slingshot, you need a body other than the one relative to which you want to accelerate. So you can use Mars to accelerate relative to the Sun, but not relative to Mars. This means if you're already in Mars orbit you need fuel(TM) to escape. One might consider a slingshot around Deimos or Phobos but I don't think that'd be helpful enough to justify its cost (could be wrong, but I've never seen it suggested in any Mars->Earth mission plan).