I recently began keeping a poker journal. A journal is an effective tool for knowledge based work. Writers, scientists, and stock traders are among the people that keep a journal of their work. I recently read about the importance of keeping a journal for traders. Many successful traders are known for keeping a careful record of every trade they have made. Trading and poker have a lot in common (perhaps I will write more about this sometime), thus I thought that a poker journal would be a cool thing to try.
An $8 hard cover ruled notebook from Staples is all you need to get started. I use the journal to record any ideas, questions, hand analysis, and calculations. I also use it to track brainstorming and rough thinking (you'll find many mind maps in my journal). Many times I've read threads where a good question is posed. Now I write these questions down so I don't forget to think about them at a later time. A journal is a very personal thing, but I recommend you to try it out and see what works for you.