BJJ black belt Steven Grossi is a huge believer in jiu-jitsu journals.
Journals can help you retain information (even if you’re a visual person). Maybe a move didn’t work for you as a white belt but works really well as a blue blue belt or purple belt. Thanks to your journal, you can go back to your journal and go over all of its fine points.
Journals are also a great way to reminisce about your fondest BJJ memories — e.g., like the day you were promoted to blue belt or the day you finally escaped the “blue belt blues” and became a purple belt.
Journals don’t need to be books, either. Many professors allow their students to record techniques using smartphones. If your handwriting is awful, you might want to consider making yourself a video archive of techniques.