Open the search terms report. Set the range to the last 30 days. Sort by cost, descending.
Work down the list asking a single question: would I have chosen to pay for this search?
Every "no" becomes a negative keyword. Every surprising "yes" becomes a keyword of its own.
Why it works
It attacks the largest, least glamorous source of waste in almost every account, requires no budget, no restructure and no approval, and it compounds — each week’s exclusions protect every week after it.
Why almost nobody does it
Because it is dull, it does not make a good report, and nobody has ever been praised for it. The full routine.