Theresa Lowe has created a "safety net" routine that will create an extra, separate copy of your drawing each time you save. Normally when you save a drawing in AutoCAD, the DWG file is updated and the previous saved file becomes the BAK file (assuming you have BAK file creation enabled). In contrast, this routine creates a totally separate copy of the DWG file, uniquely named, that will never go away until you delete it.
Start by loading the LISP file. When you enter the command Bak, the current drawing is saved as normal and another copy of it is written to a predefined location, including the directory structure of the current drawing. The only real downside to this routine is that you could forget about the backups after a while and end up with thousands of drawings filling up your drive, so you'll want to take the time to manage this periodically. Lastly, depending on how you save drawings now, you might want to edit your menus or shortcuts so that this code is called instead of just the built-in QSave command.