I did this the cheap way since i didnt want to fork out the cash for dynomat...
First my rear licence plate was rattling so i just took some spay adhesive and glued a peice of tar paper over the back of the licence plate. Stopped it from rattling and saved me 25 bucks for a dynomat licence plate thing.
Then i took out all of my trunk lining, (its not that hard) and spayed the underside of it all with "rubberized undercoating" (or somthing like that), you know the stuff that you would spay under your wheels in the fender linings to prevent rust. Well on the can it also said it would help baffle road noise, so i figured it would do the same in my trunk, and since its like spray on rubber, it would kill all the vibrations between the lining and the metal too.
So i just spayed that on the back of all the linings...and then put them all back in, since its under the lining, no one will ever see it and it wount depreciate the value of the car since it doesnt look like crap.
There was a few other spots like the licence plate frame, the whole peice slides out from your trunk, i sprayed the inside of that with it too...
Anyways it took a few hours and cost like 25 bucks, better than close to 200 for dynomat.
If you want more details just ask...