If you find your plugs are pretty solid when cold, just loosen them slightly when engine is warm. Once slightly loose, a drop of WD40 will work its way into the threads. I then gently retighten and loosen a few times until the threads start to free up. When the plugs are all out, I put a new plug in each cylinder with a drop of WD40 and make sure they screw in easily before cleaning the threads and nipping them up. Never had any problems with cars or motorcycles.
Can be a bit worrying when loosening really tight plugs, but following these steps will help. After all, it doesn't matter how tight they are, they have to come out one way or another.