And when I say back-up I don't mean just create a second copy on the same machine in case the first become corrupted. Nor do I mean putting it on a memory stick that is kept in the computer bag (and the bag gets stolen). Or in the house when the computer is stored, so when the computer is stolen the copy goes too.
Back-up means that no matter what happens to the top copy, a second one is still available.
Obviously you can just upload to some free storage space. But will you remember to do it every day? Carbonite is an excellent online service that provides the perfect back-up. You get 15 days free trial. But then the costs seem reasonable.