Sound deadeners?

I started using some stuff called FormShield (aluminium backed closed cell foam) and have found that it only makes a little difference to the sound level - however I think that once the sound gets in this stuff just lets it bounce around more than soak it up.  
I also tried some of Knox Auto Carpet's sound proof felt, and that's made a huge difference!  Just laying the two sheets you get in the rear of the van, sides touching down the centreline of the van and the rest laying partially up the sides and wheel wells seemed to soak up heaps of the soundwaves - well it sounded heaps quieter anyway!  I originally bought it for going on the front floor and firewall, but I'm going to buy more of it and put it wherever I can get it to stick to - making it stay against the walls could be difficult but I'm lining the entire insides of my van so I'm sure I can find a way to hold it there.  It would also soak up any water that gets in so you have to make sure the van's sealed from the elements.
From what I read of Dynamat it's an excellent product, but totally covering all panels isn't really necessary - strategically placed pieces can do just as well in dampening the vibrations of each panel (and hence the perceived noise level).  It's also not the kindest on the wallet! I'm thinking of using a few pieces on large panels like the roof and walls in conjunction with the soundproofing felt to see how it reacts.