The question you need to ask yourself is "what am I doing wrong?" We can provide you with suggestions, but if you don't know exactly what's wrong then you won't really know exactly what to do about it.
Personally, I had major issues with bar placement, which was causing my elbow pain. In the end, it turned out that I wasn't keeping my chest up/not keeping my upper back in extension, which would cause the bar to roll down my back a bit. In order to prevent the bar from falling off my back, I would have to support it on my back with my arms, which was causing my elbow problems. I fixed it by resetting my squat weight (which I had to do anyways since this issue was causing my squat to stall) and focusing all my energy on keeping my chest up, hard.
So, find out what YOU are doing wrong, and fix it.