In these situations I put the camera on my shoulder and just mingle for a while. I talk to people, say hi, let them know I am no creep. Pretty soon someone will ask about the giant camera you are lugging around. That's your in, use it.
I do this even when working professionally at an event. I will walk up to a group of people, shake some hands, introduce myself and then ask to shoot. Granted in the capacity of the event photog no one really gives you a hard time and generally hams it up. I do this early on, then later on people are comfortable with you and have seen you shooting all night. At this point I am able to get candid shots with out people getting upset (or paying any attention to me at all) when they discover me.