Apparently castor oil can 'work' because the nerves/muscles/something that wirks the bowel also works the uterus, so laxatives can stimulate contractions...but I doubt it would bring on labour if you wern't rather close anyway.
Nipple massage worked for both of mine (but maybe was close anyway?).
DD, was 7 days overdue, so massaged for about 15 mins every couple of hours all day, then went into labour early next morning.
DS, was keen not to go overdue, massaged every night (!) for 15 mins from about 38 weeks, a bit more closer to due date, had DS 4 days late after a nice (huh?) short labour.
I've read that nipple massage is actually proven by studies (as oppsed to pineapples...apparently you'd have to eat 40 to get enough of the chemical to do anything at all!), but really, how can you tell if baby was just about to come anyway? Having said that, will def do nipple massage again for any more kids I have.
HTH