Buy caffeinated drinks (if you’re keen on such things) and keep them in your room. Take one on the Saturday morning, and one on the Sunday morning, and at any point during the game when you feel like you could use the energy. Except, of course, a few hours before you intend to get some sleep. I’ve found caffeinated drinks keep me alive over an entire weekend of epic roleplay, but I’m not sure if it works for anyone else.
Keep your room locked (if you can) when nobody is in it. Every so often an opportunist with a lack of respect for other people’s property will make their way through the camp (if you’re using a camp) or other such venue and take advantage of any unlocked, unoccupied rooms. If your room doesn’t lock, the GM’s will most likely have a room in which you can stash your valuables (such as your wallet) if you don’t have a car to put such things in. Use this room to stash your OOC valuables to make sure they don’t get stolen.
Plan any transport to and from the venue well in advance. This will save you money on airfares if you’re traveling by plane, as well as making it easier to get to the venue when the day comes to do so as you’ll have had time to prepare for the trip.
Lure as many of your friends to come along as possible. Having people there that you know will make your time at the game a lot more enjoyable. Having said that; make sure you branch out from your group and talk to as many people at the game as you can get away with while still acting in-character. You’ll most likely find that a great many of them will be fun to roleplay with.
Your character background is a reflection on the sort of events that you want the GM’s to throw at you. Leave heaps of unresolved events in your character’s past to ensure that such things come back to get you during the course of the campaign. If you like surprises, leave unsolved mysteries in your character’s past, e.g. “I walked in on my brother’s murderer as they did the wicked deed, but I did not catch their face in the darkness. They fled through the window, and all I saw of them was a wide brimmed black hat and a red cloak.” Such a line makes your brother’s murderer an unknown person, but the wide brimmed hat and red cloak make them identifiable, should the GM’s choose to bring this mysterious murderer back into your character’s life during the course of the roleplay.
If you can, write shared background events with other roleplayers. This will give you reasons to interact with those people when you go to the game.
Make sure you involve your character in historical events and factions from the setting of the LARP. This way your character will have reasons to react to events in-game that involve the factions and historical events that you wrote them into.
The most important thing you can give your character to ensure that you have a reason to get involved in things going on around you is a reason for your character to want to be in the LARP’s setting. Why are they here? How did they get here? Why do they want to stay? What do they want to do while they’re here? These sorts of questions will give your character a firm grounding and rationale for making themselves a part of what is going on around them. Badass loners might sound cool as a concept, but being lonely during a campaign LARP session is no fun at all.
That’s all that comes to mind right now, I hope it helps.