Regrettably this isn't something I have a lot of advice for but I figured it might bring up some conversation and perhaps it's a topic I can re-visit with loads of really good answers from my next big LARP campaign. Basically the problem is this: you get a group of players (generally the most experienced ones) who've likely had a big and tiring week and in the middle of game they drop character and start chatting about their general day-to-day lives so rather than driving the LARP (as again they're most often the very influential ones) they are a big immersion-breaking clot.
Naturally part of the problem can be traced back to the players not having enough to do in-character but alas this isn't always the case. Often someone simply thought of a cool story and then that reminded another player of an interesting thought which led to a comparison with another event/movie/videogame and then everyone's wrapped up in the chatter.
I've noted that my literal presence often tames such conversations even without having to make friendly shooing motions or even reminders, but once the GM is over in the corner all bets are off. In my last LARP, which was quite small by the end of it, everyone dropped OOC the moment I'd left even if I was taking a loo break and they were in the thick of discussions. I'd pop back, they'd pick up their IC roles pretty seamlessly and we'd get back on with it. They hadn't waited for a rules check or anything but my absence had become a bit of a Tea Break indicator even if I'd specifically designated another player (any player) to try to keep them IC.
Not a terrible thing, to be sure, as in this case all players dropped in and out-of-character at will and without any real interruption, but still an amusing peculiarity of my LARP's customs.
But anyway ... ever been to a LARP where OOC conversations would tie up a number of players? What helped? What didn't? What does one do? Leave all of your ideas in the comments box below!