Clary sage (and this goes for the vast majority of essential oils) should NEVER be used undiluted directly on the skin. If using for massage, some caution should be used, at the least you should obtain a reputable book on aromatherapy, or consult a qualified aromatherapist. Depending on the oil, one or two drops to a tablespoon of carrier oil is sufficient. If using essential oils for massage during pregnancy further dilution is suggested.

Clary sage (jasmine is another suitable labour oil) will stimulate and tone the uterus and so will assist and strengthen contractions. It is usually recommended that it only be used once labour has started. (It is also a useful oil during menstruation).

To be honest, I don't think that there is a trick in the world that will bring on labour if the body is not actually ready, but it can be helped along. You might find acupuncture more useful though.