Dynamat sucks. I put some of their regular crap in my trunk and it didn't do squat. I can't find the research page but someone did a scientific style experiment between many of the available sound deadening manufacturers.
RAAMAudio came out as the best bang for the buck. It's much like Dynamat Extreme, but cheaper.
There is a really good write up on NASIOC.com. You put down a layer of mat (double it up behind the door speaker), then a layer of their Ensolite foam. Seal up any openings with metal flashing from Home Depot and self tapping sheet metal screws. Then do another single layer of mat and single layer of foam.