As a digital photographer my backup strategy is extremely important, Its a simple scenario, If my machine went down (hard drive crash) and I had no backup I would lose years of work, work that simply can not be replaced, so I take backup fairly seriously.
My system is fairly simple,. 2nd hard drive in Box (not the OS hard drive) stores master files, that hard drive gets backed up to a large external USB drive once a week, there is also a secondary backup from the master files that runs once a month out to a large NAS drive, then once a year (or so) I burn all crucial new images onto DVD.
All paying jobs go through this same system but also once the job is done they go directly to DVD labled for that job.
All my little business doc's just go along for the ride with my photo files.
I think soon ill need to come up with an off site strategy as well. probably including DVD's or tapes in a bank vault somewhere.