Oh! My! GOSH!!! How did I miss this topic? I definitely have something to share.
Journal is great tool.
I would add that you should reread it and reflect from time to time to learn something from yourself.
I prefer file in Google Docs so I be able to insert links and pics into it, and it's available from all my devices. If you use some file, It's convenient to date it backwards so new records always were at the top. I combine it with To Do list. Very powerful planning tool.
Some creative people can combine it with scrapbooking. You can draw right there, glue some photos and memorable pieces of paper like transport tickets from awesome trip.
Remember, it isn't a draft and shall stay your friend and adviser for many years so be careful to store it securely to not lose it nor expose to accidental people. It's only for your eyes, so you could feel securely to write there.
I do the same, but call it meditation =] Really useful practice. Brain does a lot of job during the day and big part of it is going at background. Too many extra unwished mental processes drain daily dose of resources like willpower or attention extremely quickly. Meditation is like a reset button on the brain. It can't restore energy like proper sleep, but can stop noisy thoughts and emotions to save some power until the end of the day.
It's crucial for ones with very active nervous system like me (and some other ponies on this forum, who wakes up at 5am and have to grind till sunset LOL)