I just had to reinstall my computer from a backup I had made on a Firewire drive. In the process I discovered that I should have been making such backups with a program called Carbon Copy Cloner, rather than numerous other programs (PsyncX, Deja Vu, etc.) that are a little less thorough about what is involved in such backups. I also learned some tips through my experiences.
You can download Carbon Copy Cloner here.
More information about my experiences, as well as some tips I learned are posted here.
Of course, it is always good to keep a backup using the $50 Retrospect Express, which is much more thorough, but it will not create a bootable firewire drive like CCC. And it is much slower.
You can easily sync up with CCC without even thinking about it. If your computer is plugged in at home all the time, you can even schedule regular backups. And it is free/donation ware. (You just need a firewire drive that is the same size as your main hard drive.) You can get cheap firewire drives here.