The most frequently useful thing I've gotten out of Overcoming Bias is not a technique or lesson so much as it is an attitude. It's the most ridiculously simple thing of all: to be in the habit of actually, seriously asking: is (this idea) really actually true? You can ask anyone if they think their beliefs are true, and they'll say yes, but it's another thing to know on a gut level that you could just be wrong, and for this to scare you, not in the sense of "O terror!--if my cherished belief were false, then I could not live!" but rather the sense of "O terror!--my cherished belief could be false, and if I'm not absurdly careful, I could live my whole life and not even know!"
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.