Probably already blogged about elsewhere, but since I just installed it again: psync is a very practical software written in Perl that makes it possible to copy an entire Apple OS X volume in such a way that the result is even bootable afterwards. With this you can, for example, very easily mirror your boot partition to an external FireWire drive and use it both as a backup and as an emergency system.
There's also a GUI available for anyone who doesn't like using the Terminal.