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!"