What are your rules of thumb?

Check boundary cases. Check extreme cases. Check trivial cases.

Use the amount of unobvious subtleties involved in a subject you know very well to remind you of the unobvious subtleties that are probably present in the subject you don't know very well.

Personally: "Check you've taken your medication, exercised, eaten and slept sufficiently before giving bad feelings credence."

More generally, when considering a question "Always google it first."

The scope of the project is about right when what you're planning to do starts to feel too small, simple and narrow.

"There is nothing so useless as doing efficiently that which should not be done at all."

-- Peter Drucker, management consultant

In other words, when you're thinking about the best way to accomplish an unpleasant task, never forget that not doing it is an option, too.

When evaluating a [scientific] claim: Extraordinary claims require extraordinary evidence. This is basic Bayesianism, without the calculations. For example, if a preprint is called Solution to the cosmological constant problem, one glance at the abstract is sufficient to dismiss it with high confidence.

When designing software (or anything else): The cheapest, fastest, and most reliable components of a computer system are those that aren't there. Specifically, this means ruthlessly pruning my original design, until only the necessary functional parts remain. The Ideal Final Result approach is quite useful there.

When designing anything new: Build one to throw away, or at least be prepared to (sunk cost avoidance). This one is rather controversial, with people arguing that Agile Development/RAD make this unnecessary. However, if you notice how often a major OSS project is redesigned way late in the game because the original architectural and design decisions no longer apply, you might as well plan for it in advance.

Being angry/upset/mad/panicked/fearful is a signal that:

I've probably done something wrong, and I have temporarily diminished reasoning abilities do to having entered flight-or-fight mode.

Mental cue in quotes.

"Have I googled this?" I still forget this one way too much.

Related: For any major life decision: "Have you put even 30 minutes of serious analysis and research into this?"

Econ:
"Am I at the frontier?" as a reminder of the production possibility frontier whenever presented with ostensibly mutually exclusive choices.

"What does the marginal case look like?"

"SXD" a reminder of supply and demand in cases involving fungibility, what affects each in this case?

Lesswrong:
"behaviors, not goals" this helps fix a variety of errors in updating models of others and self.

"Am I/Are we/they being strategic?"

"TDT" a reminder that I will act similarly in all similar situations.

A literal rule of thumb.

"If you have an important point to make, don't try to be subtle or clever. Use a pile driver. Hit the point once. Then come back and hit it again. Then hit it a third time - a tremendous whack."

If you're trying to do something you've never done before, and it involves multiple steps, something will go wrong.

Nobody is stupid, nobody is evil, and everyone is crazy.

Distrust any claim or story that is entertaining enough to be passed on based on its entertainment value alone.

What domains of knowledge are relevant to what I'm trying to do? Am I a domain expert? (The answer is almost always "no," but the point of asking this question is to emphasize that so I can't conveniently forget it.) If not, who are the domain experts? How do I find domain experts who could offer useful advice? Do the domain experts in this domain generally agree with each other, or is there substantial controversy, and if so, why?

Apply mental force to the problem. Amount and quality of thinking time seriously affects results.

I am often in situations where there would be a good result even if I did many stupid things. Recognize that success in those situation does not predict future success in more difficult situations.

Do the heavy lifting your own self.

Be willing to be right, even in the face of serious skepticism. [My father told me a story when I was a kid: In a parade, everyone was marching in line except one guy who was six feet to the right. His mother yelled, "Look, my son is the only one in the right place." I thought there was at least a nominal probability that was true. And still do.]

Be willing to be wrong and concede error. [In some quarters, there is much rejoicing when I am wrong about something. Hanging head in shame brings joy to others.]

Unreliable people are unreliable. Do not assume they operate in any way similar to ordinary, decent people. [I sometimes listen to people who I know are unreliable, and I think, "That person saying this adds significantly to its truth probability," when that assumption is known to be baseless. Much progress there, though.]

The fact that some results are unmeasured and not apparent to others known to you does not mean those results are meaningless. [Preventing future crime is good, even if you don't know what exact crime you've prevented.]

Want trumps all. [Super-high-output people virtually always are tenacious about Getting Stuff Done. Intelligence matters, but GSD is always critical.]

"Find a list or taxonomy"

For example, if you want to write comprehensively on the topic of societal problems, you might look at lists of industrial sectors, the titles and subtitles of the U.S Legal code, or a typology of crimes.

This is a very cheap way to crowdsource the availability heursitic, and to appear somewhat useful without having having any unique perspectives or good solutions.

Before I get a tattoo, I have to have continuously liked a design for a full year. This hasn't happened yet so I'm ink free, but it has been a year since I decided I wanted to get magnetic finger implants so I'm moving forward with that.

Other rule of thumb I use a lot: most times, the wrong decision is better than no decision than all.

If some step of a process which you expect to repeat can be automated, automate it, even if this takes ten times longer than just doing it "by hand."

When consistently applied over a long period of time, you eventually end up with webs of interlocking automatic steps which allow you to not only do the impossible, but iterate on the impossible a couple of times to make it look cleaner.

"Try to make it better." This is to counteract my tendency, when in conflict with someone, to default to the goal "Make myself seem right and make the other person feel bad." Which, if I stop and think about it, is never what I actually want.

Try to ask two questions every so often:

• Is what I'm doing useful (including enjoyable)?
• Is there a better way I could be doing what I'm doing?

If my inhibitions regarding a certain course of action seem entirely internal, go through with it because I'm probably limiting my options for no good reason.

I think staying classy can be taken too far if you're not careful. Sometimes doing slightly embarrassing or vulnerable things helps you build connections with trustworthy people. Sometimes, it just can't be helped. I'd say worry about being kind and respectful, but don't worry too much about things like ... crying or puking. They're not particularly classy, but not a big deal either.

Don't make an effort to be clever until you know that being naive won't be sufficient.

Arguments can prove too much as well as too little.

Selection bias doesn't explain everything, but it explains more than it seems like it should!

I notice an annoying empirical regularity. (The films I want to see at the cinema are the ones most likely to be sold out. Buses take too long to arrive, and when they do they come in twos or threes. Things go wrong most often when I'm trying to get something done in a hurry.) Chances are it's neither coincidence, nor reality magically conspiring against me — instead there's usually some mundane, obvious-in-hindsight explanation. The game theoretic analogue of this rule is Scott Aaronson's observation that a situation often sucks because its not sucking wouldn't be a Nash equilibrium.

-Hanlon's razor - I always start from the assumption that people seek the happiness of others once their own basic needs are met, then go from there. Helps me avoid the "rich people/fanatics/foreigners/etc are trying to kill us all [because they're purely evil and nonhuman]" conspiracies.

-"What would happen if I apply x a huge amount of times?" - taking things to the absurd level help expose the trend and is one of my favourite heuristics. Yes, it ignores the middle of the function, but more often than not, the value at x->infinity is all that matters. And when it isn't, the middle tends to be obvious anyway.

A good way to find flaws in arguments: Take the argument to it's logical extreme, this likely results in an absurd conclusion. Now figure out why this conclusion is absurd and see if the same problem applies to the argument in the case under consideration.

When considering an abstract argument, it helps to keep a concrete example in mind.

Criticism is not censorship. (It's amazing how many flame wars seem to occur because the participants can't make this distinction.)

• If something sounds certainly correct, check it up on Wikipedia anyway - it takes less than a minute. Likewise if it sounds almost certainly wrong.
• If I don't know why exactly someone went to his conclusion, do not assume he thinks it for the wrong reasons.
• If I can predict I will be too busy to go to gym in the next few days, do a 5-minute (1-set) exercise - this is at least 50% of the efficiency of a normal exercise.
• When I feel the drive to argue, do careful judgement on whether it's efficient to do so.
• Never blame people for their biases. If they don't understand me, and even if they are trying not to understand me, blaming the people is meaningless. It is my fault that I could not predict them and was not persuasive. Furthermore, such people are usually kind and not even being unintentionally mean, no matter how bad are the results of their actions (this also applies to extreme cases of biasedness, such as outgroup thinking).
• Don't ignore the judgement of people that appear to be basing their opinion on anecdotal evidence and are easily biased. They may or may not have a good reason for thinking that. If I don't know how did they reach the conclusion, no matter how absurd their arguments sound, they might be added after the bottom line was drawn, while the bottom line being based on reasonable evidence (this has happened at least once).

Assume that whatever social interactions I have are going to come back to me in twenty years, so try not to make my future self too embarrassed; be as polite and respectful as feasible.

This is true but tricky; Many of the behaviours that I have previously thought of as polite or respectful I've later learnt come off as detached, dismissive, or abrupt. And asking people about what politeness or respect means to them is surprisingly uninformative.

So I personally find this useful only once it's simplified to essentially Right Intent -- ie. act out of intent to improve the overall situation, not malice, spite, fear, or jealousy. Anything more complex I find too confusing to accurately apply in realtime.

From lessons I learned in HPMOR before making an important decision ask yourself "What do you think you know, and why do you think you know it?" I have found that this not only shows you what knowledge you have is sound enough to make decisions on but shows which pieces of knowledge you're emotionally attached to and would therefore lead to a biased conclusion.

The first and foremost rule of thumb that I follow is 'question everything'. I find it easy to apply and it has yielded positive results.

My ignorance and capacity for error are unending, but through great effort and feedback from others I sometimes may learn from my mistakes and make new mistakes next time.