What's the most profound bit of hiking advice you've ever been given?

Mine came in a backpacking shop,20 years ago, when I asked the salesman whether I should buy a headlamp to supplement my flashlight and candle lantern. He looked at me for a moment, then said: "When it gets dark, go to bed!" ("Idiot!!" was implied.)

That sparked a whole new approach for me, of really asking myself what constituted a "need."