Since steel and aluminum are dissimilar metals there is a current flow between them. This galvanic corrosion effectively welds the two metals together over time. Anti-size compound acts as an electrical insulator to prevent this corrosion from taking place. If you change plugs every couple of years, and don't use anti-size, the corossion causes pitting on the threads in the head. Using a small amount of anti-size (I use a very small dab) will also act as heat sink compound, filling in the small pits and improve heat transfer. Also keep in mind that the Duratec has a tapered seat for the plug, and uses no washer, so once the taper on the plug comes in contact with the head only another 1/16 turn fully seats it.
To get stuck plugs out (as has alread been said) get the engine as warm (hot) as you can without burning youself: the sparkplug hole expands more than the spark plug. It's safest to put plugs into a cold engine for most accurate torque. It's alos very important to keep the socket extension in a straight line with the spark plug, and not cocked to avoid breaking the plug.
P.S. I very recently got my Mystique and want to thank everyone on this list for tipping me off on the ins and outs of these cars.
