See the last post for how I back up my important files to a remote server. To restore them I do this:
duplicity --file-to-restore path/to/file.txt scp://firstname.lastname@example.org//home/example/bkp/home/andy/Maildir file.txt
and enter the passphrase when prompted.
Note that I can restore this backup even if I lose my entire home hard drive because it doesn’t rely on any key file – just my passphrase. This is less secure than using a private key file, but where do you back up your key file to? You know it will be lost when you need it … safer just to write down your passphrase somewhere, I reckon.