How it all works in practice
Here is my backup routine. Bear in mind that I (a) have a lot of storage space on the external hard drive and (b) am utterly paranoid about losing my writing-related files. Your own routine may be very different from mine, and that's fine, so long as you develop a routine that fits your needs.

Online sources of files
I manually run Blogger's blog backup once a week or whenever I've posted a few times, and direct the resultant files to the Documents directory on my desktop computer.

My web-based email accounts don't contain anything crucial at this time, so I don't back them up.

Netbook computer
I use this machine purely for writing, so its backups are all on-demand and document-related. To ensure that I don't overwrite a particularly brilliant paragraph from yesterday with today's dull replacement, I run a different backup job each day of the work week, putting that day's version of the files into its own folder. Each week, those versions are overwritten with the new week's version. This way, I always have the last five days' worth of versions. Since I also save files under a new name if I make significant changes I feel I might regret, this ensures I'll always be able to revert to an older version if I need to.

These backups are all directed to the external hard drive.

I also use SyncBack on the netbook to keep its files synchronized with the files on my Palm, since I do sometimes edit (or fiddle with) my writing on the go.

Desktop computer
Netbook backups: This machine accesses the netbook daily at 11:30PM and 11:45PM. The first backup doesn't keep any files that no longer exist on the netbook (thereby creating an exact copy of the netbook's directories) and the second keeps all deleted files. These are put into the desktop's Documents directory.

My Documents: Copy all files from Documents to the external hard drive. This runs at one in the morning, and includes all picture and music files, as well as my blog backups, a copy of everything on my Palm, and the netbook's backups.

Web site uploads: I copy the first netbook backup to my Web site at 1:30AM, and copy all key Palm files (datebook, address book, memo pad, task list) to my Web site at 2:00AM.

All written out, it looks overdone and excessive. And perhaps it is. But when I accidentally overwrote the final version of my first novel, I was so very glad I had the backups in place. It'll take a few hours to set up, but it's well worth it.