I started journaling 4 years ago. I have not missed one day in 4 years (except for a couple of days after a mild bicycle accident where I was sleeping in bed). I built myself a system that sends me an email every day, and the replies get stored in an sqlite db. The system accepts answers up to five days after the date (meaning, if the replies is sent more than five days after the initial email was sent, it's not filed). I did this to force myself to never reply too late.

The funny thing is, five days is waaay too long a time to remember what you have done. When I describe my day at the end of it, or the morning after, I usually remember incredible little details and thoughts. But after a day, only a rough outline remains. After two days, I usually remember nothing (save for exceptional circumstances).

That's why I started journaling: to be able to tell what happened in the past. Not just the big milestones, but all the little steps.