Mirroring is the behaviour in which one person copies another person usually while in social interaction with them. It may include miming gestures, movements, body language, muscle tensions, expressions, tones, eye movements, breathing, tempo, accent, attitude, choice of words/metaphors and other aspects of communication. It is often observed among couples or close friends. (Wikipedia)
Two people often mirror each other when they are close, as an aid to communication. It relaxes the other person and puts them at their ease at a time of stress. Interestingly, mirroring behaviour is also seen between a parent and child -- for learning, particularly in the social field. In this case, it is associated with the development and maintenance of self-esteem and a sense of worth, creating self-respect.
Ask yourself, in this picture of David Cameron Nick Clegg, whether the two leaders are developing and maintaining both self-esteem and a sense of worth as they deliver a press conference in Birmingham to "relaunch" the Coalition. As you can see, their postures are almost identical, exactly as if they are reflections of each other. Hence, mirroring. Judge for yourself.