What's the money move that made the biggest difference for the least effort once you finally figured it out?

Mine was setting up an automatic transfer of a small amount to a separate savings account the same day my paycheck hits, before it ever lands in my checking where I'd spend it. Not a big amount, small enough that I genuinely didn't feel it leave. For years I'd tried to save whatever was "left over" at the end of the month and there was never anything left over, because money in my checking account is money I will spend, full stop. Moving it out automatically the day it arrived meant I was saving without having to decide to save every time, which was the part I always failed at. Took me way too long to realize the problem was never discipline, it was that I was relying on discipline at all. What was yours, how long did it take you to land on it, and what did it actually change?