From what I understand, the "new car smell" is actually caused by the natural emissions of the plastic and vinyl surfaces in your car. If you clean them throughly enough, the new car smell should actually return. I use Vinylex for this purpose, but there are many excellent products that you can try.
My car is one year old, and friends and relatives are always commenting on how it still smells "new". Once you get it really clean, you can just use Vinylex, 303, or another vinyl protectant every three or four weeks, and it should smell new forever.