It's actually the oils in the leather that give it that smell. Here's my tips:
1. Some BMW dealerships use Lexol and Vinylex. You can get both from any Pep Boys. Not only do they recondition and clean the leather, but they leave that "new car" interior smell.
2. Try to keep the windows up as much as possible when parked even in the garage to prevent the oils from drying. Sun shields help cool the interior when the car is parked outside.
3. Prevent dust build-up on the leather. That desaturates it and dries the oils over time. That means vacuuming the carpet, too, to prevent more dust from flying around.
This isn't a guarantee that your leather will always smell new, and it's different accross certain types of leather and their colors (dye used). But I can tell you that four years of operation and my BMW's interior still smells like it came out of the dealership, without the aid of air fresheners.
It'll be one hell of a ride...