I wrote about my system
here. Basically, the more valuable the data, the further "offsite" you want to go, so my most critical data goes to
Dropbox, encrypted if necessary. If I'm online, it's updated the
second I save it, and I can recover old versions if I make a mistake or delete something I shouldn't. I can also get to my data via any web browser.
Less critical data is synced between two PCs, and least-critical data between two physical disks (of different types) in the same machine.