In general I don't often go to a party thinking I'm going to perform. It's always a possibility, of course, but I can do a half hour or more with absolutely nothing so I really just don't worry about it unless I have the urge to perform a specific routine or trick.
When I want to perform I usually just have to introduce myself as Jamie D. Grant calls it, by giving an impression ie: "I'm Christopher. I'm a Witchdoctor." This, of course, pretty much immediately leads to them asking about what I do, and that almost always leads to, "Can you show me something now?" Yes. Yes I can.
Telling people I study hypnosis yields similar results.
If I want to be more subtle about it, I just steer conversations to subjects that allow me to do "minor demonstrations" of strange phenomenon I've discovered. Like if someone brings up ghost stories, I've got routines for that. If someone mentions how close they are to someone who's there, I've got a routine for that. Pretty much, I just take the subject, pique their interest in the idea, then do a trick as a demonstration of the subject at hand. At that point I usually have a choice to either just leave it at that, or to leverage that into further performance.
Ideally I will never ask someone if they want me to perform. Almost always they ask me to do something and that's how I prefer it - it takes the pressure off of me, and also puts me in control of the situation - which for what I do is pretty essential.
Just never make the mistake of using magic as a substitute for having an actual personality.