Backing up your Mac via Time Machine is highly recommended, and super easy to do, as well. It’s really the only backup system I’ve ever found myself using on a regular basis, because it’s so simple to use and easy to set up. All you need to do is connect any USB drive to your Mac, head to the Time Machine preferences, and select that USB drive as your Time Machine backup. Mac OS X does the rest.
I was thinking, though, that since I back up my Macbook Air onto a 128 GB flash drive, it’s even more possible than ever that someone might get a hold of the drive and then be able to have all my backed up stuff on it. That’s not a huge deal for me, as I don’t keep much on the Macbook Air in terms of private stuff, but if I did, I’d want to keep those files extra secure.
Encryption could be the answer, and Mac OS X Mountain Lion makes it easy.