I recomend everyone do it at least once. (If you are doing it just once, do a one shot scenario with an established troupe at a con).
Most larps are based on The World of Darkness, set in a modern world with somewhat modern characters. However there are larps based on Star Wars, Tolkein Fantasy, Star Trek, and other settings. Find one which fits your likes and something you can dress for.
Find out as much about the people in the larp. The golden rule of new gamers apply: Never game with anyone you wouldn’t spend 4-8 hours doing something else with. If you have a larp, try to befriend some people who are in the larp before hand. (This also helps, because people tend to rp with people they know, rather than people they don’t know).
This will also help you determine the “Dress code” for the game. This is the way you are expected to dress. It determine how “odd” or in costume and makeup they expect you to be. Even if you don’t know the gamesystem or the game setting well, dressing appropriately lets you fit in immediately.
Unless you plan on sitting in the cafe all night and being beautiful or literary, larping will involve standing, walking, and occasionally running and jumping. Wear shoes you can do that in. You might be wearing a slinky dress, but black flats are always a good idea.
Find out as much as you can about the setting and rules of the larp. This allows you to play without gaffing in character (“Sure I like the Sabbat.” Other player slaps his forehead. “You know this is a Camarilla town?” Now they have to either kill you OR you have to cover) or appearing to be a total N00b. This will enhance everyone’s play.
On this note: Learn any handsigns involved in the game. This unspoken language is crucial to larping success in any larp, not just a vampire one.
If you have a character sheet, find a way to shrink it on to one or two 3x5 cards. This makes carrying your character sheet easier. Also know where each game mechanic is on your sheet, that way you will not be fumbling around looking for things. (Copy shop, reduced setting, a little glue, and some 3x5 cards…. very cool)
If the game is at night or in a variety of areas, bring a tiny flashlight. Always handy and uspecially useful if you are looking things up on your character sheets.
Find out about the game boundries and time period. Walk the boundries and familiarize yourself with the area slightly before you start the game. Walking it at noon the day before is a good idea.
If the game is outside, dress for the weather rather than your character. Just because your character is invunerable to cold and wet, does not mean you are.
Just because you are in the game boundries and time, don’t forget your common sense. Just because your character can kill an army, does not mean you can. Larpers can get attacked just as easily as mundanes.
Once you get a character, practice it before play starts. This starts with you thinking about the character. Once you have a handle on it, think about how it will speak… what gestures it might do. Then practice some things in character. This will polish your performance some and give you things that will help you stay in character.
Larpers are more actors than table top players or play by posters. So remember, you are an actor(ess). You are going to “act out” most of your character’s actions. Be prepared.
Don’t play yourself, but slightly different (I’m me, but a vampire). It only gets confusing for people and can get strange for you as well. Choose someone close to you, but different enough to be interesting.
Don’t play across type. If you are a overweight male in your 40s, you can’t play a 14 year old girl. Nope. Play something you are at least close to physically.
That should sum it up for now… Any other questions?
Additional Ideas (2)
If you do run an adventure like this, I recommend you have quite a large group and enlist the help of one or two others to act as multiple GMs as this takes the pressure off. When we play, we usually do so in the woods nearby, and restrict the playing area to about 1km^2. The multiple GMs can then more easily keep track of where the players are and can spring attacks on them.
For instance, we ran a game based on Jurassic Park. Obviously we couldn't get real dinosaurs, so we did it in the dark in the dense woods with lots of velociraptors because they are more human-sized than a Tyrannosaurus Rex and when it's dark no-one can tell the difference. There was no actual combat involved, mainly running away from dinosaurs and problem solving (trying to turn the emergency power supplies for the park online). I don't think combat works very well with LARP, it's better to create an atmosphere and scare people.
As Moonhunter has stressed, vampires and undead (or psychotic killers) make a good premise for LARP because they are so human-like and therefore require a minimum of effects.
As for organised LARPs, I don't know much, but I've found a few links (mainly relevant for the UK):
(most of them seem to be based in the South )
You could always try the relevant Usenet group...
Hope these are of some help!