My naive backup plan:
Mirror my files to an external drive. If my computer files, copy files back across!
But this doesn't prevent this problem:
What if a file becomes corrupted on my computer (e.g. a random photo or something), then I accidentally backup the corrupted file, thereby overwriting my nice backup! By the time I notice the file's corrupted, the corrupted version has already propagated into my backup and so I don't have a backup version to restore from.
How do you all deal with this? I'm on OS X, so a Mac/Unix solution would be nice!