Good soaking as already stated. If you're using a flat choke wrench there should be a hole in the top you can slide a skinny Phillips head screwdriver through, that'll give you some leverage, and a place to put a cheater pipe. Also, I know it sounds counter-intuitive, but you might try tightening it. Lots of times there's just crud in the threads and it's easier to tighten than loosen. Once the crud breaks loose it might back out OK. Best of luck with it.
"I have no idea what WW-III will be fought with, but WW-IV will be fought with sticks and stones."