this is how i get maya on my back - i should say this gets easier as she gets older. it was possible when she was 8 months old (when i got the Ergo) but it's much easier now because she helps a bit.
i loosen the shoulder straps all the way. put the belt on so the carrier is over your left hip (or your right, but it's easier for me on the left). leave the belt a bit loose, but not too loose. bring your right arm behind you and loop it through the right shoulder strap. i leave the strap around my wrist at this point. put your baby on your hip, lined up correctly in the Ergo.
bend forward at the waist, and slide the right strap into place on your right shoulder while simultaneously using your left hand at the baby's waist to scootch her onto your back. flip the left shoulder strap in place and clip the chest clip.
Straighten up, make sure the baby is square in the carrier (if needed, stick your hands behind you and slide them under her bum, bounce a little, so she bounces up out of the carrier about an inch while you adjust it). tighten the shoulder straps.
it sounds more complicated that it is, now that i type it up, but it's really pretty much one fluid motion by now.