This cron job will create a compressed backup of all the mysql databases under your account. The backup will be stored as "\daily-backup\Mon.gz" - and so forth, one for each day of the week. In this way you will have rotating backups going back seven days.

First, create the "daily-backup" folder under your home directory.

Go into the System - Cron Jobs section in webmin and paste this in as a new cron job (all one line)

/usr/local/bin/mysqldump --skip-opt -uUSERNAME -pPASSWORD --quote-names --complete-insert --extended-insert --quick --compact --lock-tables=false --skip-add-locks --all-databases | gzip > /home/USERNAME/daily-backup/sql-alldb-`date "+%a"`.gz 

Make sure to replace the USERNAME and PASSWORD with your own info.

You can set it up to run on any kind of daily schedule; I have it set to run daily at an early-morning time that I picked randomly.