How do you break loose a clean out plug?

Several possible aids (assumes METAL parts): (1)Apply Liquid Wrench or similar penetrating product - wait 10 minutes or so, try again. May need to repeat this several times (2) Tap lightly on outer plug socket with a hammer. (3) Use a very LARGE pipewrench (20" or more)-all things equal, the more leverage, the better. A length of pipe can function as an extender to provide even more advantage. (4)If all else fails, apply HEAT to outer joint with a propane torch. This will expand female part of connection and permit better penetration of lubricant. (Be VERY CAREFUL about the immediate environment here-obvious potential fire hazard.

Somewhere in this series of steps, you should see results. If not, I'm stuck. When you replace plug, use high-quality pipe dope or Teflon tape on threads.