I think the most frequently useful thing I've learned is not in the content, but in the approach. The way of breaking things down, pulling together the themes, and looking at things in a way to makes the problems that seem to be there disappear. I guess that's what's been most frequently useful: when you get stuck, trying to see if you're looking at a problem in the wrong way, since often even if you're not it gets you unstuck.
Followup to: The Most Important Thing You Learned
What's the most frequently useful thing you've learned on OB - not the most memorable or most valuable, but the thing you use most often? What influences your behavior, factors in more than one decision? Please give a concrete example if you can. This isn't limited to archetypally "mundane" activities: if your daily life involves difficult research or arguing with philosophers, go ahead and describe that too.