I make backups of my images all the time. The key for me was to start out on the right track...
o Start with the distro image
o Write it to an SDCard
o Run it in an RPi but DON'T use the expand option that raspi-config offers.
o Instead use rpi-wiggle which does the same thing, but backs off a bit rather than using the entire full disk.
o Backup the image using Win32diskimage....
o Write the new backup image to whatever SDcards you want to...
I do backups whenever there is some major change or I want to mirror an image to multiple devices.
I just did a new master after updating with the newest Firmware that supports the new memory conf via config.txt (no more bunch of start elf files). I am in the process of writing out duplicate images for the other two devices now all of the master that I backed up earlier this evening.
Using Rpi-Wiggle - http://rpi.tnet.com/project/scripts/rpi-wiggle
Backing up an image using Win32diskimage - http://rpi.tnet.com/project/faqs/backups/backingup
Note if you use the raspi-config expand you can end up with the following error if you try to write a backed up image a new card.
A sad error... which can only be gotten around by doing a lot of work in Linux.