I jot down stuff I need exclusively for the current day. An issue number. The phrase " Bob about the thing". A doodle I make in order to give my hand something to do while I attend a meeting.
The pages become completely unintelligible very soon, sometimes during the same day. It is useful - taking something out of the head and putting it in paper allows me to spend less time worrying "I must make sure I don't forget that issue, I must email bob about the thing". Once the issue is addressed, or bob is emailed, 80% of the time the task is done, and the notebook has fulfilled its primary function.
Months from now, it will have become complete jibberish, I won't be able to understand any of it. But I can still enjoy the doodles.