Ask HN: Any tips on how to relax and be very attentive during an onsite?

Just trying to figure out how people deal with over-thinking everything during an onsite interview. Despite being well prepared there's always the unknown part which throws you off. Not necessarily stress per se, more like not being 100% attentive or present while being interviewed for the whole day.

Any tips on how to relax and be 100% in it?