In Les Giblin's classic, "How To Have Confidence And Power in Dealing With People," he shows that the best way to get someone to act in a certain way is to let them "live up" to your expectation of them.  He provides several wonderful examples of people who were trusted, and lived up to that trust. The following is as good as any "truth serum" on the market today.

"A police officer was consistently able to get thugs to give him information. How? By saying, 'People tell me you have quite a reputation as a tough guy and that you've been in lots of trouble, but there's one thing you won't do - You won't lie.  They say, if you tell me anything at all, it will be the truth - and that's the reason I'm here.'"  Wow, talk about giving a person something to live up to!

Les quotes famed British statesman Sir Winston Churchill as saying, "I have found that the best way to get another to acquire a virtue, is to impute it to him."

This works not just for finding out the truth, but for all aspects of WINNING WITHOUT INTIMIDATION. My first big opportunity to utilize this method was with a person from whom I needed some specialized information. I'd used her services before and she always did a good job. Not great, but good. This time, however, she was having some trouble locating the necessary information. As she was looking frustrated and ready to give up, I looked at the person next to me and said, "I don't know if this information can be found or not, but I'll tell you this - if anyone can find it, she can." You can bet your boots she found it and now she goes out of her way for me whenever I need her help.

Gear this method to your own unique circumstances. Not just to see if it works - it works! - but to practice getting really good at making it work for you in a variety of situations. Is your significant other losing their patience quickly? Then "appreciate" (verbally) the fact that one thing about them you truly admire is their high degree of patience. Is your child feeling as though math isn't his "thing?" Then express your delight that he has such a quick mind for numbers.

Is your boss displaying a temper that has you upset? Then, as you go into their office to ask a question about an unrelated issue, just happen to comment on the fact that you always admire how he or she treats their staff as team members, not subordinates.