I do this all the time, and when I try to act or look differently, I feel extremely uncomfortable and paranoid, and have to stop.
I find that a lot of the time it has to do with my environment. At my old job, I worked there for years and I felt very comfortable with the restaurant and other employees, and I acted perfectly normal and friendly with anyone new coming in. Or when my roommates would have friends over or throw parties, because it was in my own home I felt comfortable enough, and was fine. Whereas if I'm out in public, or in an unfamiliar environment, I'm tense and pretty much unapproachable. Perhaps being somewhere you feel comfortable can help.
I have changed certain people's perceptions of me over time and by getting to know them better. Perhaps that's key to some of us, and those people who just come and go or don't want to go further than a first impression will never have the pleasure of getting to know us. At least that's how I see it. 