I was researching sound dampening and was considering making a post a few days ago.
Something you can try is Peel n Seal, its a roof insulator. Its dirt cheap and comes in rolls. You can find it at lowes ( i think). Lots of people have had good results based on what I can find on google.
Although the problem is that it is a tar based adhesive, and in very hot weather it can come lose and get all gooey. I have a black car, which means if it was in the door panels, it would get VERY HOT. Bad. I was thinking of using it in the flooring of the trunk and maybe something like edead everywhere else.
Edit: Check this out. http://www.secondskinaudio.com/ They make the best stick on sound dampening material (besides its high price)