How do I back up a running system?

["Robert S" <robert.spam.me.senseless@gmail.com>]

I have debian running on a "headless" system.  I'd like to back the entire 
system up.  Its difficult with a bootable disk without a monitor (so 
Clonezilla etc are out).  I've tried mondoarchive but it usually bails out 
before it completes the backup.

Are there any suggestions?  A simple script would be nice.