If you've ever looked in iTunes > Preferences > Devices you've probably seen what appear to be duplicate backups of your various iDevices labeled with a date. For example, you could see "My iPhone" followed by "My iPhone - Mar 23, 2013 9:17 AM." This second one is an archive that iTunes automatically created, typically when you updated to new software or otherwise made a change that caused your iPhone to start anew.
There are times, however, when it might be handy to create one of these at a point in time of your choosing, say if you're about to delete a bunch of apps to test something but might want all that data back.
Good news, there is a way! Simply hold down the Control key and click on any backup there. You'll see three options: Delete, Archive, and Show in Finder. Choose "Archive" and immediately iTunes will slap the date on there.
Control-Click on a Backup to Archive it
The next time you backup, iTunes will create a new backup for that device, leaving your previous Archive intact. Obviously if you want to delete any archived backups, clicking again with the Control key will allow you to permanently delete them, too.
...and just like that, a new backup appears in the list, leaving your old one in tact