Matching happens progressively. You don't have to immediately match every aspect of the person's body language. Start with just one aspect - say the overall posture or stance. Then gradually match the angle of the head, crossing limbs and limb movements, size of gestures, voice volume and pitch, and so on. Matching forms a continuum rather than an either/or approach. So you can experiment with any level of part matching while you gain experience and develop your skill. If someone sits or stands in a particularly marked or idiosyncratic way., consider just moving part way to matching their posture. You need not mimic the person exactly in order to establish and maintain rapport.
What about facial expressions? A person with pronounced expressions who raises their eyebrows, pops or rolls their eyes, purses their lips, grimaces and so on, will not feel at home with a passive, poker-faced person who does not betray their feelings. They expect a like response. To such a person facial movements seem normal. You will find these expressions hard to fake so stay within your comfort zone, going part way rather than not matching at all. With very overt physical positions, such as crossed and uncrossed arms or legs, it helps to allow some delay before matching so that the movement appears quite natural. You can use variations on the theme of part matching, sometimes called 'crossover matching'. For instance., if the person folds their arms you could cross your legs, or vice versa. If the other person clasps their hands you might just place one hand on the other. If they rub their hands together you can shuffle your feet. If they fiddle with a plastic cup you can click your pen, and so on. As with 'going part way', you need not match exact item for exact item.
