While the use of never-seize and higher torque will help some, your operating on borrowed time. When you have a plug loose in the hole, the compression stroke jams it against the top of the threads and the intake stroke pulls it away from the top of the threads and this constant banging will continue to hog out the threads until one day the plug will be blown out of the hole. Keep checking those plugs and if they don't stay tight after the never seize and higher torque, then assume your going to have to repair the plug threads.
People have, in a pinch, JB Welded in sparkplugs, and they have held. But if you do this and it does hold, then the plug may never come out again, even at a machine shop.
Removing plugs when the engine is warm can strip out a thread easily I don't recommend it nor does Ford. If you must do it, then BEFORE warming the engine, break the plug free and then retighten.
If you tighten plugs on a warm engine then next morning before starting the car, recheck the torque with a torque wrench. When I was drag racing my Kawasaki I had the plugs in and out all night long at the track but always checked them in the morning.
And of course, make sure that any carbon on the seating area is scraped away. That is probably the biggest cause of this problem.
I just got done putting back together my 2002 Zetec after having to pull the head and have a helicoil put in for a plug. I made a stab at documenting it which I'm working on putting up on http://www.mittelstaedt.us
but suffice it to say after doing it that I can understand why some people will swap engines with one from a wrecker rather than do this job.
Also note that Ford does NOT recommend helicoils for plug threads (at least not for the Zetec anyway, probably for most others) The reason is that for it to be done right on the Zetec the helicoil hole MUST be drilled with the head locked down on milling equipment that can allow the operator to make the hole and tap for the insert perfectly parallel to the existing hole, you cannot lean over the engine with a hand drill and have at it. Otherwise the plug will screw in cocked and not seal. The spark plug inserts (lock-n-stitch, Time-sert, etc.) don't have this requirement since they carry their own sealing ring as part of the insert. The helicoil must be installed by a machine shop that is experienced with this type of repair in this type of engine.
I would do the insert myself with the head on the engine if the plug holes were accessible. On the Zetec they definitely are not accessible. On other engines this kind of problem isn't that big of a deal and the insert kits can be used with the head on the engine.