I know you've heard the nanny say it a million times, (if you watch that show).
Get down on their level.
This works for teenagers as well as little ones. No one wants to be talked down to. You would be surprised how differently someone responds when you speak to them at eye level rather than talking down to them.
Getting below their level and talking up to them can have surprisingly good results too. Especially when interacting with a child (or even an adult) who has trust issues.
Try it out, but don't just talk to them at that level, listen too. (I know what I'm talking about, I get paid to do this).