Independant artists have difficulty getting into the Mastered For iTunes program, but if you wanted to try to deal directly with apple to get in here are the requirements directly from Apple. Even if you meet the requirements they may still referr you. On top of that, they also have payment thresholds and you may never get paid if you don't meet the threshold for your first payment. And now the requirements.
Nick Landis is now a Certified Mastered For iTunes Mastering Engineer.