I don't feel depressed very often anymore. Looking back, I can see the causes with more clarity, and have found strategies to dissipate it when I need to.
Like everyone else, I collected a lot of emotional garbage throughout my life. Some from trauma, some from fears, some from simple everyday struggles I just didn't want to face. I learned to cope by numbing myself to the bad feelings. I ignored it all in favour of the "safeness" of depression. I was so numb that I couldn't really feel good things either. It was so automatic that I didn't know I was doing it, eventually just accepting depression as something I went through, maybe caused by some chemical defect.
I started meditating a couple years ago. This helped my depression (and all areas of my life) immensely. I knew it worked, but didn't know why. Eventually it got to the point where bouts of depression were out of the norm. They were surprising. I had less tolerance for them, so I worked harder at finding the root cause, and I did! It was always the same. I'd have an experience that caused unpleasant feelings, often just a trigger that brought up past trauma, and I wouldn't process it properly, instead shutting my feelings off to avoid the pain. I still do this sometimes, but instead of ignoring it I look for a way out—some type of emotional outlet to bring the feelings up and out. Sometimes just talking for a few minutes will bring up tears, and relief. Sometimes just a hug. When alone, meditating or journaling is great. Venting onto a sheet of paper then burning it can be fun, or cleaning a closet (or your Facebook friends list). There are infinite ways to find catharsis, and every situation is different.
Processing emotions like this has changed my life. It has released the heaviness of the past, making it easier to tackle things in the present. Most importantly, I can feel things again.