Making a copy onto removeable media. The time period between dumps, and what kinds of dumps you use will depend on how busy the machine is, how much you can afford to lose, how long you want to spend on backup, and how long you want to spend in restoring from backup. Restoring from an incremental, though, usually means restoring from the full dump, then from one or more incrementals. For a system that doesn't change much, you can reduce restoration to two stages by making incrementals since the last full back up. On a Linux system, you'll probably have one of pax, John Gilmore's tar or, most likely, gnu tar.
Gnu tar has extensions for remote tape access, and multi-volume tapes.
There are two tar formats in common use. I personally prefer to use tar, because I can never remember whatthe arguments are for cpio. There are several different CPIO formats. This doesn't matter unless you want to transfer cpio archives between platforms, or you need to back up a file system with more than 65536 files on it. Restore in particular has an interactive mode that is very easy to use.