Lee Hart writes (January 2004):

Personally, I have several hundred floppy disks for my Atari 800, Kaypro
4, Heathkit H89, and IMSAI 8080 computers that are 10-20 years old.  What
I can say in general:

 - Most disks stored in plastic boxes or ziplock baggies survived.
 - Most disks stored in cardboard boxes or just their sleeves
   did NOT survive.
 - Some brands lasted better than others, but I haven't collated the
   information so as to make any kind of definitive statements.
 - If a disk cannot be read, CLEAN THE DISK DRIVE HEAD before attempting
   to read another disk! Otherwise, crap from the bad disk will remain
   on the head, and will scar and destroy any SUBSEQUENT disk you put
   in the drive! (the voice of painful experience).
 - For lack of a better plan, for each of my surviving disks I am:
        a. reformatting another blank disk
        b. copying the data from an old disk onto the blank disk
   Then I have a more recently-produced backup disk in case the
   original disk later fails.