Goldstone, it's most helpful if you start with the inside of the door panels. I used to have a Miata - talk about noise! I noticed a major improvement in NHV reduction and also gained an added bonus of real nice THUNK upon closing the doors. I found that it also helps to have more than just one layer. I don't know where diminishing returns are with this approach though.
I reccomend the door panels because it's the easiest, most noticable place to start. It also help to add dyanmat to the floor panels/firewall, the area under the rear seats and the trunk lid and walls (quite a bit of noise/resonance can come from here. If/when I have more time I'm going to try this with my w124 400e. I have come accustomed to the low NHV of my old Q45 and I find my E to be a bit noisy. Though a bit pricy, I think I'll be using Dyanamt "Super" - its their most HD product and does not require heat for proper adhesion.
If you really are ambitious you can fill your rocker panels and possibly A pillars with structual foam. Don't know if you might run into long term rust problems by blocking water passages. I do know this also helps greatly reduce NHV. My former (now wifes) Q45 has foam in the A pilars and I think some parts of the rockers as well. I read an article in SSC (Sport Compact Car) in which they did this to a late model 300zx, mainly to try an stiffen the structure - the found that there was a major reduction in NHV. The actually used foam that was for a Q45.