Here are the features I'd look out for:
- As near to fully automatic as possible. If it relies on you to press a button or run a program regularly, you will get bored and eventually stop bothering. An hourly cron job takes care of this for me; I rsync to the 24x7 server I run on my home net.
- Multiple removable backup media so you can keep some off site (and/or take one with you when you travel). I do this with a warm-pluggable SATA drive bay and a cron job which emails me every week to remind me to change drives.
- Strongly encrypted media, in case you lose one. The linux encrypted device support (
cryptsetup et al) does this for me.
- Some sort of point-in-time recovery, but consider carefully what resolution you want. Daily might be enough - having multiple backup media probably gets you this - or you might want something more comprehensive like Apple's Time Machine. I've used some careful rsync options with my removable drives: every day creates a fresh snapshot directory, but files which are unchanged from the previous day are hard linked instead of copied, to save space.
