For the past 18 months or so I've been using an app on my phone call "iMoodJournal" (I have no commercial connection with the publisher!). It has proved totally invaluable to me. It has been a diagnostic aid which helped my pdoc nail down a diagnosis; it is my early warning system because I can see if I'm trending up, down, or holding steady. It is also part of my daily routine which in turn triggers other events.
What's your experience? Do you have a favourite app? What are your recommendations to people who don't (yet) use a mood journal?