My lovely DiL often presents me with a black sack full of the children's horse riding clothes which at this time of year are VERY difficult to wash. The stains from the mud, grease, manure and whatever that yucky brown stuff is, that she puts on the horses to make them shine, makes it impossible to just throw the jods etc. into the washing machine.
I tend to fill a bucket with hot water and add a generous scoop of Vanish Gold. I let them steep for a few hours (sometimes over night) and whist I wouldn't say the stains magically vanish, it certainly makes the items easier to wash. I sometimes resort to an old scrubbing brush and fairy liquid.