Every time I start a new project, read a book/tutorial/blog, or watch a lecture, I write down notes in a Google Doc. These notes are mostly bullet notes summarizing the main idea of what it is that I've learned (Feynman Technique), and I make sure to document from which source I learned that thing. Every few weeks I'll read through this document full of notes to refresh my memory on whichever topic I choose. That way, instead of wasting time (for example) re-read a dense technical textbook, I can quickly ramp up on a topic by reading my notes. If those notes are insufficient, I'll reference my bibliography and go straight to the source. The benefit of having these notes stored in Google Docs is that I can reference them from any device. However lately I've been considering switching to writing all notes in plain text files with vi so that I could grep and find things easier later on.

I also personally retain knowledge better when I write it down. I always keep a stack of printer paper or composition notebooks nearby so that I can write down my ideas. These methods aren't always very effective, but they help.