A friendly, concise overview of some of the most important techniques for improving thought, such as identifying cached thoughts and tabooing your words, holding off on proposing solutions, recognizing fake justification, leaving a line of retreat, righting wrong questions, identifying true sources of disagreement, etc.
This presupposes some motivating discussion of cognitive biases and why it is so unnatural to think rationally.
Followup to: The Most Frequently Useful Thing
What's the number one thing that goes into a book on rationality, which would make you buy a copy of that book for a friend? We can, of course, talk about all the ways that the rationality of the Distant World At Large needs to be improved. But in this case - I think the more useful data might be the Near question, "With respect to the people I actually know, what do I want to see in that book, so that I can give the book to them to explain it?"
(And again, please think of your own answer-component before reading others' comments.)