You don't have to go there. It's not the events themselves but the way the mind reacts to them that can cause a minor annoyance to snowball into an all-encompassing black mood—good news because, while you can't control traffic or someone hogging the milk, you can change how you respond when things go wrong.
The facts, please: Downward spirals are often provoked by jumping to the worst-case conclusion. You make a beeline from a boss's critical e-mail right to "I'm going to be fired." Or you take a friend's failure to call as a sure sign she doesn't like you anymore. But the boss's complaint is probably just business as usual; the friend is simply distracted by a problem in her own life that has nothing to do with you. So before taking a flight of bad fancy, reread the boss's e-mail more carefully (you may be surprised to find positive comments you hadn't noticed before), and review all possible explanations. Who knows, maybe she was just stressed out by her boss.
Let it be: Humans are highly evolved problem solvers. No sooner do we experience a negative emotion than we feel compelled to fix it. Usually that urge goes with the assumption that there's something wrong with us for feeling blue, which only compounds the depression.
Another problem is that by trying to think our way out of sadness, we paradoxically fuel it. "The more you feed these downward cycles with attention, the more you allow them to proliferate," says The Mindful Way Through Depression co-author Zindel Segal, PhD. "The negative ideas and experiences will just multiply, spinning from one thing to two things to four things to eight things." Instead, he suggests accepting your sadness as a natural state, experiencing it in the moment, and allowing it to pass. This doesn't mean letting yourself slide passively into a deeper slump but, rather, engaging with your feelings in a mindful way. The following meditation exercise can help you do this.