If you make periodic named saves in addition to quicksaves, they may be less likely to get over-written. Also, each save is a separate subfolder in the '..\Divine Divinity\Run\savegames' folder, so it is relatively easy to zip or move saves to another location, to free up space or for backup.

In Windows XP you can zip a file or folder by right clicking on it in Windows Explorer and selecting 'Send To | Compressed Folder'. Other version of Windows, or to get more features in XP, require a separate program (freeware archive managers).