• One slightly heavy-handed method that worked for me:

    i) loosen the seatclamp
    ii) spray some GT-85 or something in the general area (optional)
    iii) make sure you've still got the saddle attached (important!)
    iv) stand on the pedals/position yourself up in the air above the saddle.
    v) sit down hard.
    iv) repeat as necessary.

    Seems to break the 'weld' enough to start twisting and and loosening (using the saddle as a lever).

    Good luck.