Nipple stimulation. It's no joke though...(I know from experience).
How to Induce Labor: Nipple Stimulation
One of the natural labor inducing methods is nipple stimulation. Nipple stimulation releases hormone called natural Oxytocin. It's good to note, however, that nipple stimulation has to be done correctly according to reliable instructions because it can lead to very strong uterine contractions if done too frequently.
In order to release Oxytocin by nipple stimulation, massage your nipples by rolling them between your thumbs and forefingers for 2 minutes. Then rest for 3 minutes. Repeat rolling and resting for 20 minutes. This usually gets the labor contractions to start. Remember that you should use nipple stimulation to induce your labor only if your cervix has already effacated.
Link:
How to Induce Labor Naturally?