I too have experienced depression on and off for a large part of my life. The single biggest factors I have found to improve my mental health are diet, meditation, exercise and sunlight!
What you eat has a major impact on how you feel. I found that by improving my diet my mental health also improved considerably. I now eat a vegan diet and have not eaten gluten or dairy products in 7 years. In hindsight it is clear now I have an intolerance to gluten and dairy which caused me many years of misery. I'm not saying this will be the case for you too, but it's worth been mindful about what you are eating and how it makes you feel.
Mediation and exercise have been a great help too. Meditation has helped by allowing me to become more aware of the thoughts which were contributing to my depression and letting them move on instead of reacting to them. Exercise is a no brainer - getting my body moving makes me feel good.
Sunlight - I live in London and in Winter its dark when I get up in the mornings (6-7am) and when I leave work (6-7pm). During the course of the day I get get almost no exposure to natural light except when I go outside for a walk at lunchtimes. After a couple of Winters of been depressed I finally tweaked last Winter that it was S.A.D (
The Seasonal Affective Disorder Association) and invested in a lightbox. Using the lightbox for 30minutes every morning has made a tremdenous difference to me during the winter. I can't recommend it enough.
Best of luck