Taboo Your Words and Wrong Questions together. There are numerous debates over definitions that I'd previously have happily engaged in, but now I just take a glance at them and shrug them off as trivially solvable by tabooing. Applied this into just about everything, from philosophy and Searle's Chinese Room to politics and "is online piracy theft". I feel it has considerably clarified my thinking (and it was my second candidate to the "most useful thing" thread).
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.