In Gina Trapani's excellent HowTo backup at lifehacker.com/147855/geek-to-live--automatically-back-up-your-hard-drive she suggests 3 folders, one backing up (same files to different folders) nightly, another weekly and the third once a month. I set mine up slightly differently, and prefer it. One folder backs up every second night. As it happens, the first time was an even numbered date. The second backs up the same files, but starts on an odd numbered date (7th Oct, for illustration) and repeats every 4 days The third also backs up every 4 days, but starting on the 9th. This way:-
There's a backup every night
There's never 2 backups on the same night. Given the size of some of my files and the time taken using ethernet over mains, I felt this desirable.
If anything goes wrong with a backup I've got between 3 and 6 days before I have to solve it.
The principle could be extended to 4 or more folders - 3 seemed a good balance between backup drive and doc folder sizes.
PS. Ethernet over mains allows the backup drive to be at the other end of the house, just in case. The house is 200 years old, unaccountably built without ethernet.