I started keeping a physical journal a few months ago. For some reason having the physical journal with me as a constant reminder helps me keep up with it.
Anyway, I've gone back and reread the beginning of that and I've gone "Oh yeah, I forgot about that," several times already. I can't imagine completely losing my memory and needing to refer to my journals for memory. But at least if I ever did go through this, I'll have these to refer to.
That's an interesting point about finding things you completely forgot about in journals.
It made me realize, if I had to reconstruct my memory from written notes, I would end up with a lot of stuff that was only important to me at the moment my thoughts were being written down.
I was listening to the Twit podcast once and Jerry Pournelle was on. He talked about his life log. Basically it was a journal in which he recorded everything - meetings, meals, phone calls, weather, along with the more usual thoughts and ideas. I tried to do that for a while and it was a lot of work.
First of all, it is a lot of work. I sometimes stay up for an hour and a half some nights just catching up on 3-4 days of activity. I've almost filled up the first one, though, and seeing the physical artifact and being able to look back on it and know that a lot less of what I consume and think about every day is slipping through the cracks into nothingness makes it worth it, though.
I'm a game designer by hobby (and to an extent by trade, especially in my past), so in reality the focus of the journal is more on design. If I come up with a new concept for a game, a new design for how cards or boards or levels should look (I just draw a rough sketch right there in the journal), or playtest a game and want to report feedback I received and ideas to change things, or report what I worked on in a given day, I put it in there.
I also add some life events, though, to put what I'm doing in context, explain why I was more or less active at times, and also as a gentle reminder what happened and when for events in my life. I put anything I do consume in there that might be considered an influencer and what I liked about them (board and video games, movies and tv shows mostly).
Finally, if I listen to or watch a lecture or read an article about something that could pertain to design, I write notes down separately, and then distill and flesh out the important (to me) notes into the journal later. The reason I do this is I can't really decide what's important or not while I'm in the middle of it, and I also write notes to lectures too quickly and without as much context, and I need to add that because the journal is intended for an audience other than just myself to read it (although perhaps not for a long time afterwards).
I don't record what I consider more trivial things like meetings, meals, phone calls, or weather, though, for the most part. Maybe if I we tried a new unconventional recipe and it turned out awesome, but probably nothing else.
As for structure, I just write the date, a hyphen, and keep writing until I'm done for that day. Then a blank line, and the next entry. Some days only require a paragraph, some days require 6 or more pages, so I don't try to limit or pad it or line it up with pages. I also put a few "Concept for 'game name'" in the top margins for a page if I have to find something again, but honestly if I just make sure I add more drawings of things, that's usually enough to identify where the concepts are at a glance. I've almost finished my first one and it will cover about 4 months of my life when I'm done.
Anyway, I've gone back and reread the beginning of that and I've gone "Oh yeah, I forgot about that," several times already. I can't imagine completely losing my memory and needing to refer to my journals for memory. But at least if I ever did go through this, I'll have these to refer to.