I came upon this four-year-old article from American Family Physician detailing ways to create cervical ripening if labor seems stalled and for inducing labor.
Along with drugs and invasive procedures like stripping the membranes, it discusses several herbal remedies, including blue and black cohosh and raspberry leaves. Although the article concludes that these methods aren't proven to be effective, it provides references of studies published in respected journals.
It also discusses the mechanism by which sexual intercourse and breast stimulation can induce labor to begin -- although in this case, induce is probably too strong a word:
With penetration, the lower uterine segment is stimulated. This stimulation results in a local release of prostaglandins. Female orgasms have been shown to include uterine contractions, and human semen contains prostaglandins, which are responsible for cervical ripening.
Breast massage and nipple stimulation have been shown to facilitate the release of oxytocin from the posterior pituitary gland. The most commonly prescribed technique involves gently massaging the breasts or applying warm compresses to the breasts for one hour, three times a day.
See the whole article, "Methods for Cervical Ripening and Induction of Labor."