Mastering = A good set of meters + a good set of ears. Even better get a mastering engineer to do it. Mixing and mastering are very different skills. I think too many people think they can do it all. Obviously if you're a one man show on a budget you do wat you have to, but for professional work, involve professionals. Also www.digido.com is one of the best places for mastering info. Bob Katz is one of the best.