Virtualmin backup strategy ??

Hi

As I'm being to setup a brand new backup server, I'd like some advises about best backup strategy ! I have two virtualmin servers to backup to that third backup server. I was thinking to use FTP (all in a same VLAN so no security issues). What is best way to do incremental backup on one week for example ? to optimise transfer and size of backups and still be able to restore without too much pain backups if needed ?

Thanks for your advices and tricks ;)

Vincèn