When lifting anything, keeping it as close to your body as possible relieves the stress on your back.
I am going to try to explain this as best I can. Neil you don't have to go frying me :lol:
Stand the bag up on your tailgate and sort of hug it.
Get one hand underneath the bag and one in behind it sort of low.
Lift the bag off of the tailgate such that your hand underneath the bag is about belt high.
Bend you knees and thrust upward with your body.
Slightly after starting the upward thrust with your knees, push upward with the hand under the bag. All you are really wanting is to get half the bag above the height of your shoulder. Not the whole thing.
Squat again slightly and let the bag roll onto your shoulder. You hands should now be simply balancing the bag on your shoulders.
Take smaller steps when you walk and when you let it down, let it slide down the front of your sholder and body.
Always keep any weight you lift as close to your body as you possibly can. Reaching out and lifting is what will tear up your back.
Okay. Now my citics can get after me. I'll shut up and read. That is about the best description I can do.