I would use the same procedure in the answer provided by mikes, but using trisodium phosphate (often called TSP) instead of laundry detergent.
It's readily available in home improvement stores, and is fairly cheap.
TSP is more strongly basic than laundry detergent, which is mostly sodium carbonate, so it's a stronger degreaser. It also doesn't foam up, which I prefer.
Since it is more corrosive than sodium carbonate, it's also more irritating to the skin, so you'll want to avoid skin contact (wear gloves and glasses), and avoid getting it on painted surfaces.
Cover the stain with TSP and enough water to produce a thick paste, brush it in aggressively, and let it stand for a couple of days.