One dozen naked virgins of each sex must plough the perimeter of the area. Seven circumambulations must be made, no one may glance at anyone of the opposite sex for the duration of the spell or it won't work.
From The Element Encyclopedia of 5000 Spells
Note: I really did try to find a picture of naked virgins plowing a field - that was well... you can imagine. So I settled for a rather mundane image of a plow.