1. Never a bid a game you haven't written. No, really. "Oh, sure, I have six months to write it" becomes "Well, I still have 2 months to write it" becomes "Crap, I only have a week to write it!!!". This is less true in a Campaign LARP, but still true. Life gets in your way. You get sick, you get a new job, your transmission dies. Once you have a deadline, the time you have to write the LARP is always X where X is less than the amount of time required to write the damned LARP.
2. Subtlety is for the weak. When you're writing a novel, there's room for subtlety. People are reading your novel to immerse themselves in your world, your head, your ideas. When you're writing a character history, there is no room for subtlety. They will be trying to categorize everything they see into either A) plot or B) background. Making something subtle immediately forces it to be B) Background. If you want them to be conflicted, tell them: You want A. You also want B. You know these are mutually exclusive. Let the subtlety come from the players so that you can be sure they at least have all the plot points.
3. There's no such thing as too much Plot. I honestly believed we had enough plot written for a weekend game, and they finished in less than 3 hours. If you give a character 5 plots, they will accomplish one in 30 seconds, fail to notice 2 of them, give up on the fourth one halfway through, and be unable to complete the fifth because someone else gave up on it halfway through.
4. Clever Game Mechanics can add a lot to the flavour of the game, but usually suck. The more simple or universal your Clever Game Mechanic is, the better. The more unusual or individual your Clever Game Mechanic is, the more likely it will be to hinder your game or get completely ignored - and if it's unusual or individual, it's probably damned important.
5. Anticipate every contingency. I learned too well the saying "No plan survives first contact with the enemy", so I have a hard-to-control tendency to just "wing it". But LARPing requires clever planning, and keen foresight. If you casually mention that they can blow up the planet, they will. If you allow for the possibility that they can escape from the locked-door game, they will. Even if you don't, they will.
6. Be flexible. The Willow is famed for its flexibility. In a hurricane, it will bend in ways that would startle the most inhumanly flexible contortionist. If a LARP GM were only as flexible as a Willow, the game would die a flaming inflexible death. Come up with plot on the fly. Rewrite rules and mechanics because they suck. Tell players things they forgot and beat them over the head with plot points (see also Rule #2).
Your LARP is a force of nature; if you think you can control it, you are horribly wrong.