I've always used laundry detergent mixed to a paste. Put it on and scrub real hard while adding very little water. After it soaks for about 30 minutes hit it with the washer again and it should get most of it out.
I use cheap cat litter. Sprinkle it on and grind it in with your foot. This breaks the litter down into smaller peices and allows it to get into the pores of the concrete. You may have to do this several times.
Ok, here's one. How do you get rust stains out of a newer driveway? Soaps or detergents haven't worked.