One thing people often do, and it does help to a certain extent is to write/blog about the books you read. Summarize the essentials for your readers, it'll serve them as inspiration for new reading material and you as a perfect refresh of what you actually read and worked through.
As you see bits and pieces of text that you want to remember, you should be internalizing those words, thinking about their true meaning, asking yourself if you agree/disagree, and thinking about how you would approach the same thing in a different way.