In high school, my friends used to call me Debbie Downer in reference to the SNL skit, and I still hear about it. I'm naturally more of a pessimist as well. Half the time I dont even think about it as being negative, it just happens to be what occurs to me first. What I try to do is since my first thought is often negative, try and think of a more positive way to word the same thing or mention something else. For example, someone will be talking about a particular tv show. My first thought is "the actor who played the dad died". Instead, I'd try to mention something positive like "that show was hilarious!" or something. Its hard to do, but at least if you can catch yourself sometimes you dont come off as all negative. As an extreme, you could always move to certain places in Europe. Apparently Americans are much more optimisitc than many Europeans.