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The biggest one that hasn't been mentioned yet:

Practice gratitude. Get a pen and paper and start writing down things in your life that you're grateful for. It can be something as simple as being grateful for food, water, and shelter (which gets taken for granted so often).

Do this for 15 minutes a day (preferably as soon you wake up) and you will notice your outlook on life improve. Religious people do this in the form of prayer, and it has been scientifically proven to increase your happiness.