Use a backup system that automatically backs up your data, and then nags at you if the backup fails. Test to make sure that it works.
For people who don't want / can't run their own, I've found that Crashplan is a decent one. It's free, if you only back up to other computers you own (or other peoples' computers); in my case I've got one server in Norway and one in Ireland. There have, however, been some doubts about Crashplan's correctness in the past.
There are also about half a dozen other good ones.
There have, however, been some doubts about Crashplan's correctness in the past.
Links? I use Crashplan and would be interested in learning about its bugs.
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