There are certain tells that can give away that someone is lying. For instance, when my dad lies he gets all bug-eyed. In fact, many people do that.Link, Link to feed of Little Brother Instructables
To keep from giving yourself away the first thing you want to do is keep eye contact with the person you are talking to. Unless, you are one of those freaks that never make eye contact with anyone. Then, whatever you do, don't make eye contact.
The best way to make eye contact is to focus on an imaginary spot somewhere to the left of their head around their left ear. This will seem like you are looking straight at them.
Keep a neutral and relaxed posture and don't do anything that would make you seem nervous like shake or scratch your neck. I like to stay relaxed by singing Irish folk songs to myself. The kind of song that goes on and on and on.