The easiest choice. Install the Auto Config Backup package, and enter your subscription information, and rest easy knowing it's being taken care of on your behalf. Sit back, have a cup of coffee, and read on to see what the other guys have to do.
Create a script on a server/pc to pull the config, it wouldn't have to be much more than this:
wget -q --no-check-certificate --post-data 'Submit=download' \ https://admin:pfsense@192.168.1.1/diag_backup.php \ -O config-router-`date +%Y%m%d%H%M%S`.xml
Obviously, replace "pfsense" with your password, and substitute the IP of your pfSense router after the @. You could run this periodically with cron to make regular backups. Note: The above is for a pfSense router with the WebGUI using HTTPS. If you are using http, you may need something more like:
wget -q --post-data 'Submit=download' \ http://admin:pfsense@192.168.1.1/diag_backup.php \ -O config-router-`date +%Y%m%d%H%M%S`.xml
The details of this approach are covered elsewhere on the web, and it isn't really recommended, but you could also do something such as this: