Lots has been written here about gratitude journaling.
In spite of knowing about the benefits, I've never managed to make the habit stick. I do, however, already have a very strong habit of responding to email and keeping my inbox under control.
That's what pushed me to build Email Notebook, which I'm hopeful will facilitate other folks' writing habits, too.
It's beta quality at the moment, but my email is on the contact page and I plan to put some effort into feature requests if there are any common themes. Already on the roadmap is to improve the variety of prompts, touching on some of the specific practices outlined by @david_gross.
If you do give it a try, please report any bugs you encounter :)
From the link:
Is this for a writing habit, or reminders to gratitude journal? (Also asking because of the post title.)
I originally built it with gratitude journaling in mind, but pre-launch feedback from folks suggested that this tool would be useful for people that wanted to maintain some other kind of journal. There were also some folks that wanted e.g. short story prompts, so I architected this thing to (eventually) support a variety of prompt types. I'll try to make the homepage copy more clear!