I have always used yarrow in diviniation, but it can be used for much more than that.
Yarrow is a feminine herb, ruled by the element of Water and the planet of Venus. It can be used to dispel melancholy, negative energy, lingering sorrow, or depression. As I mentioned, it may be used to aid divination. It is a good remedy for colds, and opens the pores to purify the blood. It is also said to prevent balding as a hair wash. It is used in rituals and spells for healing and hand-fasting. It is also used in spells and charms to draw love, banish negativity, and promote peaceful relations. (Especially between married couples.)