As prmaple pointed out, this is a reason version control is used. If you feel like diving into it, I recommend something like git or mercurial, as you can create a local repository on your machine. However, if you do not want to mess with the version control stuff, you may want to skim through this post (viewtopic.php?f=4&t=8271
) which provides a couple plugins for non version controlled backups. I've never used them before, so I am unsure how well they work, but it might be a good resource for you.