In controlled environments, you can say something like 'Roll this die. If it comes up 6, lie to the question, if it comes up 1-5, tell the truth.' It monkeys with your confidence interval, but it gives the respondent the 'out' if they give the socially unacceptable answer. I can't recall the details, but there was a case (possibly dealing with illegal logging) where they tried this and still got 100% reporting no illegal logging.