Tired of forking out for expensive sets of nut files?
Simply detune a string enough to slide a 30mm square of 400 grit wet/dry (dry) half way under the string, hold the wet/dry around the string, tune up slightly to provide enough tension to nearly trap the wet/dry under the string, then "sand" the nut with it toward the tuner for the perfect break angle.
I don't know if that's a common hack, but I thought of it myself. I've only done it once, and it seemed to work OK. 👍