Cover heavily with lime and pour on some kero on till its nice and wet, leave for a week or two, broom up and hit the area with strong detergent. That's what I've been told by a bricklayer.
I've never been patient enough to try that approach, I usually just rip the guts out of the stain by liberally shaking caustic soda over it and misting lightly with just enough water to get the caustic to dissolve. Pressure wash off an hour or so later. Don't get it on your skin/plants/grass/pets.