How to make plans?

You're welcome to read through my incomplete draft of a book on planning. I attempt to answer most of the questions you've asked. I hope to make more progress on it in the next year (though in the short-term this is will take the form of writing blog post again, e.g. here [LW · GW] and here [LW · GW]).

My searches for other resources showed very little on generalized planning, everything was domain specific (business planning, wedding planning) with the exception of all right book "Planning for Everything" which I think falls a little short of the title.

My guess for the most common planning mistakes:

1) Not having an actual plan, only a goal. Essentially, just saying "I want to be X", and then waiting for it to somehow magically happen. As opposed to researching how people actually get from "here" to "there", what kind of tasks they do, which skills they need, and actually practicing those skills. In other words, not making the first step, but instead waiting for the "right moment", which somehow never arrives; or if it does, it will find you unprepared.

2) Expecting the whole thing to happen in one big step, as opposed to setting up your activities and habits so that they keep drawing you in the desired direction.

For example, if you want to get fit, a typical failure is to buy an annual ticket to a gym... and then never actually go there. (Unlike the previous example, you have actually made the first step. But then you wait for the second step to happen magically.) A more successful plan would be to simply start doing push-ups every morning; and perhaps think how to reward oneself for doing so.

Or, if your goal is to become a writer, a typical failure is to start writing your big novel... only to end up a few years later with hundreds of pages of horribly written text, which obviously doesn't have a future, but the sunk costs are breaking your heart. (Now the problem is that you have skipped a few necessary steps.) A more successful plan would involve reading other people's texts and writing exercises, at specified time every week. (Similarly for computer programming.)

This may be somewhat obvious, but I'd assume optimism biases (inside view, planning fallacy, maybe competitor neglect if it's the kind of plan where competition is involved) play a big role in many if not most plans that don't work out, as well as failing to bulletproof the plan initially using e.g. murphyjitsu/premortem.

A less obvious one would be aborting a project based on noisy data causing the expected value to temporarily drop; which could be prevented by predefining clear unambiguous "ejector seats", as alkjash mentioned in their Hammertime sequence [? · GW].