Paranoia

Lab books are important but my electronic data even more so. So have your data backed up in multiple places, using multiple methods! This is what I do:

1. A hourly backup, using Time Machine, to an attached hard drive. This covers the "oops, I need that file back" as well as the "crap, my hard drive crashed" scenarios.
2. A weekly bootable mirror, using SuperDuper, to a hard drive stored in a file cabinet. This covers the "need to get back up and running RIGHT NOW" scenario.
3. A twice-weekly backup, using JungleDisk, to Amazon S3. This covers the "need access to my files from Timbuktu" and "holy shit, the building burned down" scenarios. (JungleDisk also archives previous versions of files, in effect providing a partial backup of the Time Machine drive; it's the lack of a TM mirror that currently makes me most uncomfortable.)

Excessive? No way. If I could--easily and affordably--have hourly versioned backups to the Internet and five-minute backups locally, I'd do it. But then, I also photocopy my field data books.

Update (March 16): I've added DropBox to the mix. This may replace JungleDisk in the future, but for now they're both running. This gives me the "hourly [or better!] versioned backups to the Internet" capability noted above.