we used to use pencil until a salesman from Shandon Lipshaw came in and
showed me their cassette labeler. It's called the Cassmark. It was a
lifesaver.  Permanent labeling that is legible and uneraseable.  They
were even able to customize it to my specific labeling needs. You can
label on the regular part of the cassette as well as on the side if you
need to put a lot of information on the cassette.