I usually act like I don't care if I get the job or not during interviews (as in, I'm happy where I'm at, and if you're going to lure me away you'd better make it worth my while). That way, they don't get the vibe that you're unhappy at your current job, which of course can indicate a number of potential undesirable things. It's when your over anxious, and are obviously trying hard & reaching... that raises warning flags. Plus if you're stressing out just talking to somebody, what does that say about your capacity to handle various workloads?
I've been pretty successful. Granted, there are bosses out there that want you to constantly be sucking up... and they don't like my approach. But it works both ways; I don't want to work for them anyway. And chances are if I did, they would take my constantly NEVER sucking up the wrong way, and start looking to get rid of me. Those are the kind of people you don't want to work for. So during interviews... they think they're screening me, but little do they realize I'm doing the same.
And I've had an investigator tell me to my face (after 2 hours of being 'under the bright light' so to speak) that he "couldn't read me for $h*^". So, I make these people think what I want them to, while I screen them checking for my own requirements. I can be pretty manipulative... I have this power to make people want - whatever I want them to want. Power of persuasion, or suggestion, who knows. Or maybe they just bend to my iron will.
Oh btw, don't let them degrade you, or get stupid/cute. "Describe yourself in 3 words." ... "I... am... good". ..."How did you prepare for this interview?" ..."Well, I did a lot of thinking, about how I might answer a bunch of random questions... that you might ask".
It's all one big silly game. At the end of the day I seriously laugh at ALL of it. Stress? What's the point. Might as well laugh a bit now, as well.