Herbs that Increase Milk Supply in Breastfeeding

I was told to take fennel and fenugreek in order to increase milk supply. Is it true that these work? And if so how do they work? I was also told to take brewers yeast.

I don't know that the mechanisms for herbs increasing breast milk have been researched. Anne McIntyre, in "The Complete Woman's Herbal" (Henry Holt publisher) states that fennel "has been used since the time of the ancient Greeks to promote secretion of breastmilk in nursing mothers, and when taken by the mother the volatile oils will pass to the baby to soothe digestive troubles."
Other recommendations by Susun Weed in "The Wise Woman Herbal for the Childbearing Years" (Ash Tree Publishing) include Borage leaves, Blessed Thistle leaves (Cnicus benedictus), the nourishing herbs Raspberry leaf, Nettles, Alfalfa and Red Clover and the following foods: apricots, asparagus, green beans, carrots, sweet potatoes, peas, pecans and all leafy greens.