The WD might help, and can't hurt.
If they have not already broken off, then you may want to try some 'percussive' force. Carefully tap the wrench with a hammer.
As Tox said, alternating with some tapping in the tightening direction can help too. Use minimal socket extensions. Rotational force is your goal, as lateral force can shear the plug off. A pneumatic wrench would be ideal...
Once you do get them out, I have had excellent results with anti-seize. After 30K miles they come right out, and despite the heat, some residual remains on the threads.
I have since decided that the aluminum particals in any excess anti-seize paste that melts away may not belong in my engine. Perhaps not really a problem, but why chance it.
I have now switched to using a heavy molybdenum grease, which has a much higher temperature resistance (more grease remains on the threads after 30K). Molybdenum grease is typically marketed as "assembly grease" for engine rebuilding, or as "brake grease" for sliding surfaces on brake components.
This stuff really holds up well, and the plugs go in and come out easily every time.
One of my friends swears by silicone grease for this purpose, but I have not tried it. It just seems too thin to stay put for the long haul, but the temp rating is high enough, and I have an open mind about it...
1986 300E 5-Speed 240k mi.