I was blessed (cursed) with being born into a theatrical family, so my experiences dealing with public speaking and interacting with others started early, and were well-developed.
Which is not to say that I'm extroverted, per-se. I hang out in the middle, and in my personal life I need very few people to really keep me going.
But some things that I feel help when out in public:
1) Smile. This is something people forget. Just and easy, natural smile. (Not like my avatar. That says "serial-killer" if you lead off like that . . .)
2) Be prepared with a few things you know you can fall back on for conversation. This is context oriented, but if you are somewhere to talk about a specific book, then have your elevator pitch down pat, have a few moderately successful authors or books that you can compare to for style, theme, or whatnot, and have a couple of inspirations for the book at hand. By this I mean actors, or people in your life who do things like your characters do. Just funny anecdotes, really.
3) When out around peers, engage with them. If it's a bunch of other writers, do things like compliment covers, or titles. Don't lie or flatter or anything, just find something you can appreciate, and be complimentary.
Being out in public is (or should be) just giving yourself permission to be a bright, friendly you.
Good luck!
