Rampant self-promotion time.
I've got an archive of LARP advice that's primarily aimed at GMs just starting out, but some of it can be useful for veterans, too. The site focuses on logistics and costuming, with some bits about character development and the occasional rant.
http://www.skaro.com/larpdex.html - Unsolicited, but Useful Advice for LARPers.
Enjoy!
- Johanna