well you can go all out on sound deadening material like Dynomat or even better, RAAMmat 60... but honestly, the stuff i used was found on the side of the road :pface it's called "roofing material" or something like that... it's a dense tar based, 4mm thick, and is sticky on both sides... my buddy found 8 rolls of it on the side of hte road, each roll covering about 25sq.ft, and each roll weighs about 20 pounds...
people say this stuff stinks up your car.. it might for the first week, but after that the smell goes away

i used it to sound deaden my entire trunk and get rid of any rattles in my interior panels. it's REALLY maleable and can be cut up into the smallest pieces for those little rattles you want to get rid of... looks like this (sorry for the large image but had to show SOME detail):