I've just added some interests to my LJ user info, and one of them is "immersive parties". Some theory, from an AIM conversation:
Flyers say "this party is organized" and get you a line when you open.
The entry to the party must be active in some way. Making the participants do something to enter the created world gives them a stake in the world. For example, SCA events do this by having a pile of T-tunics for visitors to put on.
You need texture. By this I mean that in order to experience a created world as "real", there has to be more to it than meets the casual eye. If the first level of further examination reveals created world rather than real world, the created world becomes real.
To maintain the texture, an immersive party at a convention needs at least one staff member per 50 square feet.
It helps tremendously to have a PA system.
An immersive party is like a crystal. The molecules in solution want to become part of it, but you have to be thorough and clean with no distractions to keep a single lattice.