[dakull]

I know this is highly anecdotal but for me reading the works of Nietzsche, Camus, Cioran, Kierkegaard et al does help in moving me a couple of steps back from getting myself into that deep dark hole from which it's really hard to climb back.

I guess reading (not necessarily just philosophy) can have a cathartic effect and thus relieving some of the symptoms of depression.

And clearly, having a good therapist to talk to when nothing else works.

This may sound cliché but the "trick" is to try being aware of that threshold and when you pass it take some action or better yet stay away from it as much as you can using whatever works for you.