The online TFS documentation describes this.
If you are using full SQL Server, you can use SQL Server Agent to schedule database backups, otherwise use the Windows Task Scheduler (through one of the SQL command line tools). But you need to write a SQL script to perform the backup, this is described in detail in the documentation above.
(There is no simple command line, because simple installations have different requirements for complex ones, for example, on a large instance, backups can take many hours, and the problems of synchronizing backups become complicated.)
Additionally [edit 2010-08-24] the following TFS PowerToys have been announced, which include backup and recovery tools (for non-standard TFS installations). See Blog Post: " Backing Up and Restoring a TFS Server "
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