I think the answers will be useful, even if they don't exactly represent the set of "most frequently useful things from OB" but instead the set of "most frequently useful among the very memorable and surprising OB posts".
Maybe Eliezer asked about the first set fully expecting to get answers from the (still useful) second set.
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.