Settings > Preferences > Backup tab > Click Verbose Backup
This will time stamp a copy of your file every time you save.
Update:
AutoSave allows to automatically save the currently open files based on a timer schedule (default is 1 min) and/or upon the application losing focus. The plugin offers a couple of options to save the current (or all the files), selecting only the named ones, accessible through a menu.
[Via Lifehacker]
Update2:
BEWARE: If the Custom Backup Directory you set to is deleted/moved, then the backups won’t happen! No error is thrown notifying you of the dead link!