dynomating or sound dampening???????

i dont like to people to be able to hear me when i roll up next to them, or when they are outside of my car and my car's doors and windows and all are closed. i did get the original dynomat done in the doors, and to be honest am not sure if it does any good or not, but what i'm wondering is if it is ok to go over the existing dynomat with extreme dynomat, or something of that nature. I'm going to buy the stuff myself from ebay or somewhere, and want to do it myself (it cant be that hard). how do u install this stuff, and price aside, what works best??? what i really want is to just keep the sound inside the car, i dont have any rattles or anything. oh, i also am just referring to the inside speakers, i dont have anything in the trunk yet. thanks for any input.