Via a link on IRC, I stumbled upon the blog of the mathematician Terry Tao. I noticed that several of his posts contain useful rationality advice, part of it overlapping with content that has been covered here. Most of the posts remind us of things that are kind of obvious, but I don't think that's necessarily a bad thing: we often need reminders of the things that are obvious.
Advance warning: the posts are pretty well interlinked, in Wikipedia/TVTropes fashion. I currently have 15 tabs open from the site.
Some posts of note:
Be sceptical of your own work. If you unexpectedly find a problem solving itself almost effortlessly, and you can’t quite see why, you should try to analyse your solution more sceptically. Most of the time, the process for solving a major problem is a lot more complex and time-consuming.
Use the wastebasket. Not every idea leads to a success, and not every first draft forms a good template for the final draft. Know when to start over from scratch, know when you should be persistent, and do keep copies around of even the failed attempts.
Learn the limitations of your tools. Knowing what your tools cannot do is just as important as knowing what they can do.
Learn and relearn your field. Simply learning the statement and proof of a problem doesn't guarantee understanding: you should test your understanding, using methods such as finding alternate proofs and trying to generalize the argument.
Write down what you've done. Write down sketches of any interesting arguments you come across - not necessarily at a publication level of quality, but detailed enough that you can forget about the details and reconstruct them later on.