The idea that that you shouldn't internally argue for or against things or propose solutions too soon is probably the most frequently useful thing. I sometimes catch myself arguing for or against something and then I think "No, I should really just ask the question."
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.