Save the file in every useful format to a floppy-disk and then make a copy of this disk. These are your master-copies, for use only in emergencies or when you need the file in one of those formats. Keep saving finished files to this disk in the same way until it's full and start another one, gradually building up a library. Don't forget to check the backup copies are all okay before storing them. This is the mind-numbingly tedious task of listening to the same song several times, but if you need backups, you need to know you can rely on them.
Floppy disks have their flaws, but they're a lot more reliable and durable than tape.
Don't forget to use the Verify procedure if your sequencer has one. You might also want to note such things as the tempo, key and time-length of the song, any changes you made to the arrangement of chords, which patches you used on each channel, the date you programmed it and so on. If you can, save this as a text-file on your 'emergency' disks. It's also handy to keep a file-copy of the lyrics here too. Every now and then programmers like to have a furtive listen to their files and perhaps give themselves a little pat on the back.