I enjoy stoic philosophy -- it helps me to stay positive in the face of negative things in my life (or things that I could choose to take negatively). I have been reading the classics -- Marcus Aurelius, Seneca.
Although I am not religious, I also really enjoy a Catholic prayer that goes something like this:
Lord give me the strength to accept those things I have no control over, to change the things I can, and to know the difference between the two.
One bizarre form of negative thinking I find myself trapped in is running scenarios of negative things that may or may not happen and my response to them. I developed this habit as a result of my job as a police officer. I am constantly analyzing every situation I am in and what could go wrong, how I am going to stop or change it. Instead, when I catch myself doing this, I decide not to think about it and move on. The cyclic negative thinking does me no good.