People like and trust people that are similar to them. We have a mechanism in our brains that catches people’s small movements when we speak with them, so even if we are not consciously aware of it, all these small details do matter.
To make mirroring more natural:
Do only moves that feel natural to you.
Use variations in amplitude.
If they make big gestures, use a small one.
Use lag time.
Wait a few seconds before you move to the mirrored position.
Once you build rapport through mirroring, you can lead the body language to your desired state. The person you are speaking with will most likely follow you. This is very useful to change a negative emotional state to something more positive.
When a person you talk to has negative body language (anger, anxiousness, etc.), mirror it, and then lead to one that’s more comfortable for you.