300winmag gave some good instruction, and I would just add that if you are having trouble putting it on you should be careful. You may want to consider either carrying less weight or conditioning your upper body a little so that you don't injure yourself. I used to carry more weight than I was in condition for, and my body took a beating.

For frenchie, an A70 at 40 lbs doesn't suprise me. I used to carry an A60 at 35 lbs all the time, and that's only cause if I put more than 35 lbs in it it became extremely uncomfortable.