Assuming the threads aren't totally shot and there is a way to unscrew the pedals, leverage is key: Sometimes the wrench you use just isn't long enough. I like to slide a pipe or an old seatpost over it and push on that for more torque. On one occasion when no pipe or seatpost was handy, I slid the end of the wrench inside the head tube of an old frame and used a whole bike for leverage. I had to have a friend steady the bike I was working on, but it gotthe job done.