For this reason, it is necessary to be relaxed, and pay attention to some things, like:
An equal handshake
This is a tough one to call when greeting someone for the first time, but if you can mirror their grip it avoids any dominant/submissive vibes.
Relax into your chair
The way you sit conveys a lot of subtle information to the people on the other side of the desk. So don't take the seat like it's Old Sparky, instead use a moment to get comfortable. If you look relaxed, it'll encourage your interviewer/s to feel at ease in your company. Just be careful not to take it to extremes, and kick back like you're at home on the sofa. Flipping the chair round and straddling it is also perhaps just a little too cocksure.
Maintain eye contact
Keep it true and steady, but remember to blink. To avoid that staring-like-a-serial-killer mistake, form a mental triangle on your interviewer's forehead and make sure that your gaze doesn't drop below eye level. Any further south and things start to get a bit intimate, an interest in their mouth may persuade them to think you're hitting on them.
Steer your body
Crossing your legs loosely is fine if it makes you feel happier, especially if you're wearing a skirt, but if you can 'point' at the interviewer with your knees or your feet it shows you're focused right in on them.
Use your hands
If you can be physically expressive as you speak it shows a certain confidence in the stuff you're saying. Use your hands to roll out your answers or give shape to your ideas, and at the very least your interviewer will think you know what you're talking about.
But remember, try to stay relaxed. The purpose is not to have the appearance of a robot, but avoid some gestures that may show negative qualities.