Bob Katz FTW. Here is a really instructive section from the intro to Mastering Audio.
Mastering requires an entirely different "head" than mixing. I once had an assistant who was a great mix engineer and who wanted to get into mastering. So I left her alone to equalize a rock album. After three hours, she was still working on the snare drum, which didn't have enough "crack." But as soon as I walked in the room I could hear something was wrong with the vocal. Which brings us to the first principle of mastering: Every action affects everything. Even touching the low bass affects the perception of the extreme highs.
Mastering is the art of comprimise: knowing what's possible and impossible and making decisions about what's most important in the music. When you work on the bass drum, you'll affect the bass for sure, sometimes for the better, sometimes worse. If the bass drum is light, you may be able to fix it by "getting under the bass" at somewhere under 60Hz, witch careful, selective equalization. You may be able to counteract a problem in the bass instrument by dipping around 80. 90, 100, but this can affect the low end of the vocal or the piano or guitar - be on the lookout for such interactions. Sometimes you can't tell if a problem can be fixed until you try; don't promise your client miracles. Experience is the best teacher.