Obviously, it is always a good idea to backup your database to a spare system disk, RAID, or to another slice on the system disk. But I think it is equally important to backup the database to something offline, i.e. tape.
If the tape hasn't been initialized before, you will need to add the keyword init into your syntax. You will only have to do that once. Again, it's a fairly straightforward process. Here is an example.
# isql -
1> use master
1> dump database MYDB to mytapealias with init