I find that you need to explicitly state your intentions ahead of time. Otherwise, people assume the worst, and get defensive right away. Then communication closes down. So my method is this: before I even ask the question, I couch it in nonthreatening terms. Usually, I start with a simple compliment. It doesn't even have to be relevant: "Hey boss, that new policy seems pretty neat. What made you implement it all of a sudden?"

I find this is effective even if you actually do want them to change their decision. Come in with a compliment, then try to lead them to recognize the flaws in the decision themselves.