As some of you who are also active on bicycles.se may know, I broke my collarbone in a bike crash last year, and it's now got a titanium plate in there. I'm not sure whether it's the plate or not, but some backpack straps now seem to press rather too much in the wrong place. My commuting backpack is worse than my good 40l daysack, for example, and I don't think it's just the weight-bearing hip belt on the latter. Adjusting the shoulder and chest straps makes little difference; the discomfort comes back soon.
A little testing with fingers or rolled up gloves suggests that pads above and below the collarbone would help a lot, but I'm not sure how to keep them in place. I don't want to simply glue bits of foam to the straps as they might need some adjusting, but simply wedging them in when I put the pack on doesn't work as they creep out. I'm also not certain that foam pads like this are the best idea, so am open to suggestions.
Here are two example packs: The blue one (right) is a hydration pack I use for running and mountain biking and the dark one (left) is the one I mention above for commuting etc. In the bottom left picture you can see that the strap that holds the chest strap stops too low; in the bottom right I'm pointing to where I believe the pressure point to be, but it would be hard enough to tell precisely even without minor nerve damage. As I'm tall and rather broad-shouldered, small one-size-fits-all packs like these are a less than perfect fit anyway.