I have read a number of articles and seen studies that state doing a few light sets after your heaviest actually inhibits maximun strength development on a given exercise. I start lighter keep adding weight until I reach my heaviest sets, then go on to the next exercise.
I think that your training properly, maybe you need a bit more recovery? As of right now the scientific community does not know what causes DOMS-delayed onset muscle soreness. The current theory is an inflammatory response to exercise. We do know that if you continue to train as you are, you will experience less and less soreness, but your gains will not be lessened.