Derek Sivers on the benefits of a daily diary and topic journals:

You can’t trust distant memories. But you can trust your daily diary. It’s the best indicator to your future self (and maybe descendants) of what was really going on in your life at this time.

Truth. I also love his idea for a Thoughts On journal:

For each subject that you might have ongoing thoughts about, start a separate “Thoughts On” journal. Whenever you have some thoughts on this subject, open up that file, write today’s date, then start writing.