Second geekmom! My midwives will not use castor oil to induce except as an absolute last resort. Definitely try all the other things she mentioned. They're all more fun anyway. :-)
In a word - don't. It can be dangerous for your baby since it often makes them poop in utero (meconium aspiration can land newborns in NICU for several days or longer with pneumonia). Drink lots of water and do lots of walking if you're trying to get labor started. Have sex. Stimulate your nipples. Take a hot shower, bounce on a birthing ball, do squats. Don't take castor oil.