Another way to hoist pack onto back....works for scuba rigs too....
Place the pack on its back at your feet, the top of the pack pointed toward your shins, shoulder straps and hip belts splayed open, to the sides. Kneel down and reach through the shoulder straps and grab the pack towards it's back. Hoist the pack straight over your head, then straighten your arms and let go of the pack. It will fall right onto your back. Make sure everything in the pack is secure before doing this but it works! It's a symmetrical lift that allows use of both arms.
IMHO, this is absolutely the best way. Your back is straight, no twisting of shoulders, no potential for strain. I found out about this when I was healing a rotator cuff injury and it really saved the day.