I've heard good stuff about PB Blaster.. I haven't ever used it first hand, but some kind of WD/Liquid Wrench/PB Blaster/Kroll soaked in there, little heat, and a BFW outta have that bugger out of there with little trouble.
Some people replace those plugs with 'innies', that way when they need to pull 'em, they can take 'em out with just a big breaker bar or ratchet.
