sound dampening

I am sure this has been beat to death here, but....I did a search and didn't come up with too much.
What would be the best and most economical product to use for sound dampeneing in my car? I am not looking to enter any competitions or anything, just lose the rattles that seem to accompany any kind of nice system.
I have heard good and bad about most. I have also heard about using roofing material and that it was not such a good idea. While I am not looking to use that, why is it not recommended?
Thanks for any suggestions