I've posted before about FutureMe.org, where you can send emails to yourself dated in the future (which I am loving, by the way).
Now there is a way to remind yourself to keep gratitude journal.
Gratitude160.com is a free service that texts you reminders about being grateful. You can choose how many times the service can text you each day, between one to four times. When you receive the text, just reply back with what you're grateful for at that moment. At the end of the week, the site sends you a report listing everything you said you were grateful for.
My texts have been something tangible, such as a good lunch, to intangible, such as grateful for my family or a beautiful day.
The site asks for a donation of $10 per year for the bandwidth and technology they use. It's voluntary, so if you have it and you enjoy using the site, please make a donation.