I keep a daily journal. Beginning of day: Two things that I'm grateful for. End of day: Two things that went well that day, two things that could have gone better. Each "thing" is usually only a sentence or few long. I find that going back through the end-of-day sentences every so often is useful for doing 80-20 analyses to find out what seems to be bringing me the most happiness / dissatisfaction (at least as judged by my end-of-day assessments).

I write down one line (about 80 characters) about what things I did each day. Originally I intended to write down "accomplishments" in order to incentivise myself into being more accomplished, but it has since morphed into also being a record of notable things that happened, and a lot of free-form whining over how bad certain days are. It's kindof nice to be able to go back and figure out when exactly something in the past happens, or generally reminisce about what was going on some years ago.

I wrote this on the topic - might help with the habit of keeping a book -

http://lesswrong.com/r/discussion/lw/mpz/making_notes_an_instrumental_rationality_process/