Red raspberry leaf tea, castor oil, and nipple stimulation are the three "usual suspects" that come to mind. The lastmentioned did not work for me; and the second one there is by all accounts REALLY unpleasant, whether or not it works in a particular case.
If we can use the term on this forum, orgasm is generally regarded as quite helpful. It does not necessarily require, um, the act by which the baby was initially conceived, if there are other means you are familiar with
Another thing that is often regarded as helpful (and I had it done myself, and believe that it did help since DS#2 was born about 8 hrs later) is to have a knowledgeable midwife or nurse-midwife, or *possibly* a knowledgeable OB/GYN but I'm not sure how many of those there are :>, "sweep the membranes". Basically they do an internal and use a gloved finger to gently separate between the inside of the cervix and the amniotic sac. This usually stimulates contractions and often labor; however be aware that it carries all the usual (small but nonzero) risks of having an internal exam done late in pregnancy, plus there is some chance that even with a skilled operator (and especially with an OB/GYN :>) that the membrane may rupture and THEN you're in a whole new ballpark, the "it's been X hrs now since my water broke and my insurance company says I have to go to the hospital and be chemically induced" type thing (if you are not going into labor on your own). So having the membranes swept has both pros and cons; I did it, but can certainly see why some choose not to.
I can say that one thing that STOPS labor when you are due is to be locked out of the house by your 3 yr old, have to get up on a ladder and climb through the bathroom window and then get stuck half-in half-out headfirst-down for about five minutes til you can talk the 3 yr old into bringing you a kitchen chair to support yourself on as you finish climbing through the window. DS #2 was *going* to be born on his due date, I am positive I was in early labor, but that little escapade pushed things off a coupla days
Good luck, have fun,
Pat