Acupuncture.

Chiropractic treatment (for me it was to open my pelvis because baby hadn't engaged, but I went into labour less than 7 hours later at 41+1).

Massage therapists can perform some massage on "induction" points, like acupuncture.

Female orgasm.

Walking.

For me - I had several acupuncture treatments every two or three days in the last three weeks of the pregnancy. They were to prepare for labour, but as time wore on the treatment became stronger on the induction acupressure points. (I'm aware that there's a point on your ankle which, when pressed on either in massage or acupuncture, can bring on contractions. Most therapists won't touch your ankles until you're well over 38 weeks).

Anyway, at 41 weeks I had acupuncture in the morning, a massage in the afternoon followed by an hour-long walk in the evening. The following morning I had chiro treatment, and within 7 hours I was in labour. I think a combination of all of the above did it in the end (and the fact that I was past my EDD of course )

I've heard of castor oil doing the trick but I don't think that's really based on any evidence, and just opens the bowels. *TMI* My sister did the castor oil trick and says that diarrhea and simultaneous waters breaking is not nice