Good advice from Derek Sivers on keeping a daily diary:

It works best as a nightly routine. Just take a minute and write at least a few sentences. If you have time, write down everything on your mind. Clear it all out. But if you miss a night, make time the next morning to write about the previous day.

— Benefits of a daily diary and topic journals by Derek Sivers

I also like his idea for topic journals.

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.

— Benefits of a daily diary and topic journals by Derek Sivers