Fill the plug recess with releasing fluid and leave overnight then try again. Once it's moving mop up any excess fluid before removal.
This, and make sure you use a correctly-sized 6-point socket. Also a nice long breaker bar if you can, it's a lot easier to apply gentle but increasing amounts of torque using a long bar gently tan it is heaving away on a short ratchet handle.
I know that opinions vary on this, but on spark plugs in older engines that are vulnerable to water ingress I use a tiny smear of Copaslip on the upper part of the threads to prevent seizing. Others will say you shouldn't do this as the copper grease will melt migrate down the threads and contaminate the combustion chamber but it's never happened to me because I don't splurge the stuff all over the place.
