I saw a program on tv about the issues with our factory spark plugs and how they are prone to break if your not very careful when removing them, they said to spray a little carb cleaner at the base of the plug let it sit awhile, then crack them loose and spray a little more carb cleaner, let it sit awhile and repeat this process until they come out, personally I think I would use Sea Foam instead, that stuff eats carbon build up like nothing else and the way I would do it would be, to Sea Foam them, crack them loose, spray some more Sea Foam, turn them back in, crack them loose a little more, spray more Sea Foam, turn them back in, crack them loose a little more than the first and second time, spray Sea Foam, turn them back in, and keep working them back and forth, working the Sea Foam down into the threads and beyond.
Best of luck and take your time.