Tried that 'magnet trick' for finding studs behind drywall, actually works

I saw a video where a guy used a strong neodymium magnet to find nails in studs. Thought it was total junk, but my stud finder died on a Reno job. Grabbed a magnet from my truck, taped a string to it, and dangled it. Found every stud in that wall in about 3 minutes. It sticks right to the drywall screws. Now I keep a few in my bag for backup. Anyone got other low-tech tricks that actually save time?