To me, it comes down to three key things:
1. Have clear and correctly falling shadows, and give some thought to their colour.
2. Soften outlines of objects as they recede into the distance.
3. Soten colours too make them paler and maybe admix a little blue or turquiose.
By the way, I don't use an airbrush-oils and a fine brush are my preferred medium. You can see these three principles in action at
http://www.behance.net/Galler...ils/338334