Automatic Backup
You can enable the automatic backup option from the Configuration screen, which can be accessed through a left trackball movement or by pressing C on the keyboard:

Once enabled, every time the app closes it will copy the database as it was at the end of your usage session on the SD card.
Backup Importing
The very same backup can be imported in Tag-ToDo-List by pressing
Import backup from SD under the
more... section in the app menu. You have to be sure; by doing this your old to-do list will be overwritten by the backup. Make sure an SD card is inserted, whether a backup is made or imported.