like stated,
dynamat can be used if you're really serious about getting rid of any excess sound coming from/into the car e.g road noise, system noise like zilli said.
There is a post i saw in the DIY thread a while back that someone posted about soundproofing the doors just by using thick duct tape to cover up any holes in the steel frame, this will cause greater bass if you have aftermarket or even stock speakers in the doors, although it can potentially block some sound.. to a certain degree..
you could even use some foam from clarkerubber, ask for subwoofer lining and sound proofing.