I'd loosen all the strings to relieve the neck of the tension. i hope you have hardshell cases. and make sure to pad the head of the guitars with lots of styrofoam peanuts. I'd also try to insulate the guitars from the humidity by wraping them in plastic and bubble wrap. I might even try to shrink-wrap the entire case (with the guitar in it of course) just to be really sure, but that might be overkill.
maybe you should also get insurance on them, just in case.