the best way to get into a depression is to constantly think about "me,me and me" AND the best way to snap out of it is to start thinking about others, how can I help others.
This may sound really shallow at first but seriously this does work. I would recommend taking baby steps in this regard. Start with opening doors for elderly people etc upto helping a co-worker on some work who isnt doing too well at work. Small measured goals which eventually change your thinking.