What are your strategies to retain something that was recently learnt?

When I learn something new, I try to find relations to existing ideas or topics in my head and to map them together. It helps me to reduce the cognitive load as well as see, for example, if I solve something similar I could just reuse "it"

Also, I inmediately try to find a way to make it useful for me. This is not always the case because I tend to read more rather than "do" more but it helps me create even more relations in my brain.

Also, from time to time I revisit the topic and do the same steps as before because in the meantime I could have learned something that might be useful for the first thing.

Also, if is really important or something really nice for me I try to simplify it and teach it (Feynman technique style).

Taking notes and mind-maps have not worked for me, I feel that once is written in notes (digital) or in xmind I tend to really forget about it.

It is very much depending on the topic. What usually works well for me with technical stuff is just doing the thing. Write a little program, play with it, crash it and repeat.

For supposedly superficial knowledge and mathematics etc. the good old presentation is very helpful for me. At the moment you try to condense and present it, everything changes. What was obvious or clear becomes difficult to tell and this are the bridges that will help a lot later.

To quote one of my professors in loose translation. "I don't know much about that topic yet, maybe I should make a lecture for it."

Retention by creating course material is quite powerful and can be very deep.

Use it.