Maybe you're not lowering the bar correctly? You don't have to lower the bar all the way down while standing completely erect. You can resist to a certain point then let it kind of fall but absorb the weight with your legs. It's hard to explain but this video should help: http://www.catalystathletics.com/med...hp?videoID=357
If your shoulders are extra sore or you're in pain try to use jerk blocks if you have access to them.