Any kind of journal, but especially emotional ones. Less about what you did during the day and more about how you felt during that day.
I've tried to keep a journal so many times, but always abandon them. I have always thought that if I got to that point where I would write daily about random stuff, I would be able to write other stuff too (like, say, one of my many projects of a novel?), but I guess I have to go one step at a time. Plus, when I read older entries I can see things about myself and therefore try to fix them, feel good about them, etc, so it's good for the soul.
How do you follow through? Inconsistency is one of my many flaws.
I feel like this will be something like exercising or eating veggies. Harder at first, very much needed and healthy on the long run.
PS: I have gotten into the habit of writing down the title of every novel I finish, and the month I finished it in. But then that only takes me like half a minute to do, and only do it every now and then (when I finish my readings). At least I remember to do it every single time, and do not postpone.