For me, this is a matter of building good habits to make sure that I'm ready. I recently slipped into a 6 month dysthymia that snuck up on me, so I've put a lot of recent thought into this.
Habits I'm building:
Stay on antidepressants or mood stabilizers (I can't use certain antidepressants) even if I am not currently in a depressive state.
Because I am vulnerable to ruminative spirals (cycles of depressive thoughts that reinforce each other), keep up Cognitive-Behavioral techniques of examining the logical validity of each thought and check each one against reality (no, I am not unwanted by all because there are people who love me). This is useful for getting out of ruminative spirals once they start.
Keep socially engaged. This one is tricky for me because I'm on the extreme end of introvertedness, but I use reminders on the SuperBetter app to help me keep in touch.