I saw a video Jamie Hyneman made where he went to a comicon event (I think) and he just simply walked right through. He didn't stop, but he was friendly and said hi to everyone he came across.
Despite being so famous, he never created a crowd, and he just kept on walking and saying hi.
I've applied this to the chatty Kathy's at my work and it works every time. Just a quick "hey there!" without stopping. It never fails. Every once in a while I'll stop so I don't look like a jerk, but it works!