A lot of people keep a diary. This is a time-tested way of ensuring future historians have juicy rumours to read and write books about. A journal is like a diary, but it's meant to be more serious and less saucy.
I've been keeping a journal for some years now. It's been good in several ways:
It's a private place where I can vent entirely without any inhibition. As long as I don't leak the content, I can write anything there, things I couldn't share even with my closest friends and loved ones. In my journal I don't need to be fair or balanced or diplomatic; if calling my boss a poophead helps me, I can do that in my journal. More importantly, apart from name-calling, I can be open in my journal about my hopes and dreams, and speculate as freely as I need to about all sorts of crazy ideas. If I want to fantasize about writing my own Debian installer, my journal is where I'll do it. In my journal I don't have to worry about people misunderstanding me, or attacking any vague, half-developed crazy ideas, and ridicule me about them for the next several years.
My journal is a safe place. (This is one of the reasons why my backups are encrypted.)
It's a place to keep an external memory. One of the the things I put in my journal is a stream of consciousness while developing. This allows me to answer questions of the form "what the bleeding heck was I thinking then designing this software" with quotes from my younger self. Perhaps more usefully, this can be applied to debugging as well: tricky problems often involve a lot of data to be kept to fully understand what's going on, and a written journal is a better place for that than the brain.
A bug tracker is usually not a good place for this, or not the full stream of consciousness. Most of that stream is necessary for the process, but other people shouldn't be flooded with all of it, only the actually relevant parts.
I also put all sorts of other bits of information into my journal. In fact, over the years it has developed into a personal knowledge base, where I can find a bunch of things that are relevant to me, but not necessarily easy to find online. As an example, what's the command to do TOTP on Linux that works with Amazon? (
oathtool --base32 --totp, not too difficult to find but easier in my journal.)Some information is private in nature, such as who were the people I had that interesting lunch with at that conference that one time.
There are a myriad of tools for keeping a journal, or personal knowledge base. There's wikis of various flavors, plain text files in git, online document services, etc etc. You can keep it on paper as well, though that makes grepping harder. As I prefer to not pour my heart and soul into a service run by other people, my journal is an ikiwiki instance that I run on my laptop, which renders a static HTML that is served via Apache on my laptop (and only to my laptop). This satisfies my needs for ease of use and privacy.
Using a wiki engine for this is nice, because linking adds a lot of
power and can make finding relevant information faster. Using the
ikiwiki inline directive, which produces pages by collecting other
pages based on a pattern, I get pages for particular people (link to
the person, the person's page includes the page linking to them),
topics (e.g., projects), tags, and more. Quite nifty, and I'm afraid I
can't show you.
Keeping a journal takes a bit of effort, of course. It also takes time for a journal to become useful: having diary entries for a week probably doesn't help much. Having them from a decade changes this in a qualitative, not just a quantitative way. Do you remember what you got your loved as a present seven years ago? I don't even remember what I got as a present last year.
Give it a try. It doesn't have to be perfect, but you need to keep doing it. Report back in a comment below in four years from now and tell use if it was helpful to you.