@TrystanLea posted this a little while back.Backup module usb-import.sh script (beta)
You can use
sudo du -a / | sort -n -r | head -n 20
to find the largest files (probably logs) - it takes a few minutes.
To make the files smaller (deleting them can be a bad idea) use
truncate -s 0 /<path to file>
[edit] as long as the big files listed are not the data files!!!