During my working life, taking notes in an ordered, relevant fashion has been quite burdensome. It seems to be essential to planning not to rely on the fragile human mind, so it is evermore needed to be agile note takers. How could I, so late in my life develop this discipline?
It's a habit. Do it when you don't want to and it will come naturally to you.
Knowing the relevant notes to take depends on the speaker.
If there are powerpoint slides, you'll almost always want the exact same thing as the slides. Ask the speaker for those slides, or copy them exactly.
If the speaker is a good one, the speech or presentation will be properly arranged. Check out the introduction and write notes from there. Then check it with the conclusion.
A poor speaker won't arrange their speech. In this case, listen for main arguments. Most of the time, someone makes an argument and brings up points to support those arguments. Write down the arguments. Sometimes the supporting points too, but personally I prefer to memorize those. You'll have to sort your notes into something proper later.
If you have some smart phones or tablets, it may be a good idea just to take notes in your electronic device which you have with yourself most of the time. It is so easy to edit your notes and put check marks besides every note. Even more, you can make to-do-lists just to manage your daily work and also put your important dates in your electronic calendar and benefit from the reminder tools and etc. There is no need to have very expensive smart phone or tablet. The cheep ones on which you can install note taking, to-do-list, calendars and email organizing applications will help you to achieve your goal to organize you life with note taking and organizers, the way you like.
You are not wasting ink when you are taking notes. Overtaking notes is always better. The real danger is in missing the moment, not taking notice of something important and letting it slip away into the past.
Let the critical part of your brain make the keep or not keep decisions when you go back and clarify what those notes mean to you.
A key to developing better note taking skill is to sharpen your observation skills. I would analyze the methods of Sherlock Holmes. A fictional detective in the stories written by Athur Conan Doyle. I read a book called "Mastermind, How to Think Like Sherlock Holmes" by Maria Kornikova.
"Sherlock Holmes used strategies that led to clearer thinking and deeper insights than most. He possessed unique methods of ever-present mindfulness, astute observation, and logical deduction." (Amazon description)
The key takeaway is that you can practice and improve your observation skills.