Cayugad mentioned the Slick50 ONE grease - it is THE best stuff I have used. I apply it to the plug threads, filling the threads and perhaps a touch more, and then snug the plug without torquing it. You can buy the expensive stuff, it may or may not work as well and some of it certainly creates far more mess. The only precaution I would offer for using ONE grease is to be careful to get none on the powder side of the plug, even though the sound of gurgling 209 primers is almost funny enough to make it worth the experience.

Good stuff and relatively inexpensive.