Depends on how you want to back it up, and if you are backing up the database only, or all the files for the forum. Are you going to be storing the backups offsite on a different server / storage place?
I personally have a VPS with Backupsy, and use that to store all my files on there. I installed Rsnapshot on there, and configured it to connect 4 times a day for full incremental backups. I also have another script which drops a copy of the database each night and copies that onto there as well.