The 36 Strategies: #10 Have a Sword in a Smile.

Second only to Sun Tzu's The Art of War, the 36 Strategies have shaped Asian thought. It's important to study strategy if only from the viewpoint of being able to recognise when someone is trying to manipulate you.

Here is a good article that give an overview of the entire 36 Strategies:

The Pragmatic View, A Pragmatic View of the 36 Strategems.
http://cardinal009.blogspot.com/

... and now to #10. An all too common one that we might find used against us.

10. Have a sword in a smile

You ingratiate yourself with enemies, inducing them to trust you. When you have their confidence, you can move against them in secret.