Shut up and do the impossible, or lesser levels of 'dont run away from the problem'
In particular, the AI box experiment applied to persuasion in general. I make a lot more progress in argument when I focus on changing the persons mind, rather than picking apart their flawed arguments or defending mine
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.