Dave Hamilton, writing at The Mac Observer, recently pointed out that you can also archive backups. To do this, go to iTunes' Preferences, then click on Devices. Right-click or Control-click on one of your devices, then choose Archive. The name of the backup in the Device Backups list will change, to contain the name of the device and the date it was last backed up. The next time you sync your device, iTunes will create a new backup, and retain the old one.
Interestingly, when I checked my backups, I found a number of older ones, with dates, that weren't there before. I suspect this feature is new to iTunes 11.0.3, the latest update to iTunes, because I'd not seen these older backups before.

