The amount of information you receive every day is staggering. News articles, books, online lessons, how-to manuals, and reports. The list goes on and on. I am sure you will find it impossible to retain most of it. Sure, you don't need to remember all information you have received. In fact, most of the information needs to be actively forgotten, but some information is worth remembering. This is what I believe you can try to retain more information.
1. Choose what you want to retain mindfully
Sherlock Holmes equates our brain to that of a little empty attic. "Only a fool," he continues, "takes in all the lumber of every sort that he comes across." Suppose you haphazardly add items to your brain. You might be unable to get the valuable knowledge or risk it getting jumbled with other things. This is a pretty practical analogy. Instead of believing that your brain can retain an unlimited amount of information, think of it as a limited resource, a wet hard drive. This way, you don't haphazardly try to remember every piece of information. Sure, our brain can store a crazy amount of data. But, as you gain more and more knowledge, retrieving concepts, principles, facts, or events gets harder and harder. In short, be mindfully selective in what you put in your memory.
2. Understand deeply.
Before you try to retain any piece of knowledge in your mind, you need to understand that particular knowledge. It is easier to maintain something in your memory if you understand them. In fact, some concept does not require memorization, once you get the idea, you will remember it. So, understand the knowledge you try to remember deeply.
However, you can still forget about concepts in the long run. That leads us to the following way to retain what you have learned.
3. Use Spaced Repetition Software (SRS) like Anki.
Once you understand the subject you try to learn, move it to Anki. Not all materials need to be Ankified. Be really careful about what you want to remember. Constantly review the decks. Remove all items that are no longer relevant to your goals or intentions.
4. Use memory tools where appropriate
Memory tools are excellent for memorizing things. Loci, peg, visualizing, etc. They are all great and have been used for a long time. However, mastering these tools is not a trivial matter. So use them when even Anki is not helping you.
5. Write or teach about the subject
Writing about your subject or teaching others will quickly show where you have gaps in your knowledge. We often believe we have understood or remember a concept, only to find that we can't really explain it well. By writing or teaching about the subject, you will retain more of what you have learned.