I've read more than a few posts where if/when the plugs start squealing during removal stop turning and squirt a bit of Kroil, PB Blaster etc into the treads and wait a few minutes. If the tapered seat is off the head any penetrating fluid should seep down and into the threads quite quickly.

Not necessarily disagreeing with the pre-treatment of injector or fuel system cleaners before plug changing but I do question how effective that can really be? Just curious as I say.........

Although I do use Techron Concentrate in the recommended doses every 3K miles, add it every oil change in fact.