In one of my long-term PvP LARP games, we've done a lot of work trying to balance the system we're using, either by banning certain "classes" or "powers" or by altering their specs slightly to make them less "overpowered" so that people who choose to go this route don't have an automatic advantage over the other players. At what point does the crusade to provide balanced powers and whatnot for all types of characters tip over into nerfing cool wooj to the point that the players spend more time missing what they don't have available anymore than enjoying what's left? Essentially what strategies can you use to balance a game fairly while keeping it fun for everyone?
Balance in Roleplaying games is an illusion. It doesn't truly exist.
Even in Tabletop, using point based systems, balance is illusory. The best balanced game out there is probably Hero System 5E. But, given 100 points and no character disads, I know guys who will be pulling off 16d6 ranged killing attacks, and others struggling to get 6d ranged killing attacks. And I know guys who will do more with those 6D attacks than the guys with the 16D attacks...
In LARP, the issue is even worse.
In non-contact LARPing, it's pretty much the same issue as Tabletop gaming - some will make better use of the mechanics than others. But, because so little is resolved mechanically, the charismatic and/or brilliant players will get much more accomplished.
In Boffer LARP, some people are inherently better at the various physical actions than others, so any balance in the combat mechanics is usually much overwhelmed with the various skills of the players.
The only real test for class balance is the standard three-fold test for Tabletop:
I substantially agree with @Aramis's answer. There are some important caveats though.
Balance, in a LARP, is not about power. Balance is about activity. Make sure everyone's choices allows them to provide a real contribution to problems they choose to be able to solve. No one player should obscure many other players at what they choose to do.
The critical thing to accomplish in your balancing act, especially if adapting from a table-top RPG, is to remove "I win at everything" abilities. The beauty of a socially focused larp is that characters who win at combat are useful and marginalized. Allow people to "win" at combat, but make sure that they require the other players to function.
Having played in a 3.5 derived larp for a couple years, the first thing to do is to not play in a 3.5 larp. Barring that, don't allow all the sourcebooks.
Instead, find a minimal set of rules that allow players to express what they want to do. Players express what they want to do by showing that their characters are good at those activities. By having simple rules, there will be less conflict between those who choose to read the rules and those who don't. By avoiding splatbooks, you avoid the rules-based aggrandizement that people who are willing to invest the time into system mastery almost naturally get.
Have simple, flexible rules that can handle all manner of conflict, rather than martial combat, and that prohibit anyone from winning at more than one thing, and you'll have a solid basis for a larp.
It's possible to balance an RPG. How much work you want to spend doing it, or how important you think it is, is up to you.
The thing to remember about balancing an RPG is that you're working with a mechanical rules system. In order to achieve game balance, codify the rules, as if it were an ordinary board game, and figure out how each power or character build plays out. Most RPG authors are so interested in the theme of the game that they don't spend a lot of time thinking about the nitty, gritty mechanics of the system.
I would strongly recommend looking to video game balance as a model. My experience is that competitive online games are almost universally better balanced than tabletop RPGs. This is because the nature of the medium makes balance more important, and imbalance more glaringly obvious. You can play 4-8 League of Legends matches, each one consisting of dozens of individual battles, in the time it takes to have one battle in D&D. Consequently, it's a safe bet that LoL is better balance-tested, even if D&D has been around for years.
An article on gamasutra that might be of use to you: http://www.gamasutra.com/view/feature/2843/applying_risk_analysis_to_.php
A couple of important points:
One of the tricky things about RPGs is that the game encourages you to bend the rules. If you want a truly balanced game, you have to curb this tendency, and let the characters' powers dictate how well they fare in an encounter. If you introduce things like movable scenery, hazards, et cetera, make sure they have codified rules, and they're considered in the game balance. Since there's no way to simulate everything out there in the world under one rules system, you'll have to make judgment calls about fairness versus freedom to improvise.