I use Google Docs revision history.
True version control systems like SVN and GIT are too complex, requiring knowledge of the command line, and are really designed for collaborative teams, working on dozens of different files, all potential at the same time. They're overkill for writers.
I use Microsoft Word for writing, and every time I save Google Cloud Connect uploads my revision to Google Docs. I get backup and revision history for free.
If you don't use Microsoft Word try SyncDocs. SyncDocs works like Dropbox, creating a folder that is synced automatically to Google Docs. It also saves revision history.