Not that I'm one for changing plugs too often... but what works pretty good on threads that tend to stick is to take a regular (graphite) pencil, and do like you're blackening the threads, just twist the plug while holding the pencil point to the threads, over as much of the thread as you feel is necessary. In other words you're applying graphite, a good dry lubricant, to the threads. And pencils are always around.
This also works good on locks or anything which is out in the weather, too.
But you know what? Most manufacturers would have us changing the plugs way too much. My other motorcycle (Honda Valkyrie) is supposed to have the plugs changed every 7500 miles, when they're still new looking! A decent plug should go 50,000 miles with no problem at all, especially with unleaded fuel. My car has had its original plugs in it for 175,000 miles, and runs great, and passes California smog tests, no problem. I admit, it's about time to quit putting it off, and replace those ones, however. (I would have changed the damn things if only I could find, or at least see them. I know they're in there somewhere...) I have an old airplane that has not had new plugs for the 22 years I've owned it, and runs real good. And they were used when I got the plane; I just keep 'em clean and gapped to the specs.
But that's just me.