Reading "Diaminds" holds the promise to be on the track of making me a better rationalist, but so far I cannot say that with certainty, I'm only at the second chapter (source: recommendation here on LW, also the first chapter is dedicated to explaining the methodology, and the authors seem to be good rationalists, very aware of all the involved bias).

Also "dual n-back training" via dedicated software improves short term memory, which seems to have a direct impact on our fluid intelligence (source: vaguely remembered discussion here on LW, plus the bulletproofexec blog).