Try a change in temperature, i got a CPL stuck on my lens when i put it onto the lens in a warm media centre, then spent a day out in the cold, couldn't get it off, went back into the media centre and it came off once it warmed up a bit. I would be careful making the change in temp too drastic as you might get condensation in the lens (which goes away after a few hours).
You can get rubber pads used for undoing stuck lids, they works a charm on most things.
Not sure about other ideas, possibly a short sharp twist to try and undo it, as opposed to a constant force maybe???