I use Dropbox for backing up automatically. But it only backups up the .story file AFTER you exit Storyline 360. Storyline keeps a hard lock on the file so Dropbox can't save it until after Storyline is closed.
What I like about dropbox is it does the versioning for you. Every time you save it creates a new version. Yes you have to get in to the habit of closing Storyline every once in a while. I find that happens naturally for me anyway. I close Storyline to move on to other projects so it's never open longer than when I'm actually working on the file.
If I'm working on a project for a long time, I make sure to close Storyline and let Dropbox backup the file. It's just like clicking on Save, but it also versions the file for you.
If you have any questions of using Dropbox with Storyline please feel free to reply here. I'll do my best.
--Will