I have three methods for you to consider. I think they are all better than attaching directly to shoulder straps.
1. get something like this chest harness that attaches to a backpack: http://www.backcountrysolutions.com/ It lets you quickly get the camera ready, and holds it in a better, more stable/comfortable position.
2. if you are wearing an outer layer with a full zip, and a pack with a waist belt, then you can try this. Lengthen the camera neck strap until the tip of the camera hangs below your waist. the pack's waist belt seals off the bottom of your zippered layer, forming a big 'kangaroo pocket' that the camera can rest in. Cons are if you start working up a sweat inside, and if the jacket is loose there is still a lot of wiggle room. However, all the weight is taken off of your neck.
3. use a carabiner to clip the camera strap to the haul loop of your pack. when you put the pack on, put your head through the camera strap. Depending upon your pack (if its big enough...) the strap will be lifted off of your neck and the weight will be on the pack.
*Clipping your sternum strap over the camera strap helps stabilize it a lot.