Rock salt works very well but I can't remember the rate.  Worked at a feed store as my first job and he had us spread it on the gravel lots.  Stayed put better than I imagined it would.  Most was put under a fresh layer of gravel as we spread.  Move yard merchandise, spread salt, spread layer of gravel, put stuff back.  Wasn't bad work and forklift and tractor did most of the work.