Here are some sample commands you could use to backup your database, depending on what database system you're using. You may have to modify these commands for your particular setup. Replace the $VARIABLEs with appropriate values for your setup.
mysqldump --max-allowed-packet=32M -u $USERNAME -p $DATABASENAME > backup.sql
The value for max-allowed-packet should be the value you've set in your MySQL configuration file, and should be larger than the largest attachment in your database. See the mysqldump documentation for more information on mysqldump.
pg_dump --no-privileges --no-owner -h localhost -U $USERNAME > bugs.sql