started sending a short thank you note within an hour of every interview and one recruiter mentioned it unprompted

Started sending a two line thank you email within an hour of every interview instead of agonizing over a longer one the next day. Nothing fancy, just one specific thing from the conversation and a line about next steps. Had a recruiter mention it by name in a later call, said most candidates either skip it or send something generic days later. Costs almost nothing and apparently stands out more than it should.