Excellent question. I have found it extremely helpful either to type out or to write out by hand key claims and arguments. For almost 40 years I have carried around 5 by 7 inch cards in which I have written out parts of different texts that I update and go over continuously. I regularly cull the cards as I approach new texts or arguments, on top of which I keep journals of philosophical ideas. I also suggest sometimes re-reading multiple times parts of philosophical texts almost to the point of memorization. I still remember vividly the first text that I felt I "mastered" or practically memorized, and that was Richard Taylor's book Metaphysics, especially the chapter on God. I found (and still find) his writing lucid and engaging. On the assumption that you might still use old fashioned hard copies of books, I recommend marking them up, filling the margins with comments, counter-points, and the like.
I hope some of this might be helpful.