I've degaussed them with a Radio Shack (Tandy) "heavy duty" video tape
eraser. It takes about 20 to 30 seconds. Generic magnets work as well but
you run into the issue of not "demagnetizing" so much as magnetizing all
one way :-) To "fix" this I knew an MIS type person that put a magnet on a
stick, chucked it into a cordless drill and then spun it while lifting it
away from the tapes. Worked great and didn't overheat! 

--Chuck