It took me a long time to realize that mixing is truly an art. Dre and Timb swear by their engineers, and I'm just starting to learn the basics. Sound On Sound's web site has lots of info on this topic.
EQ cannot be emphasized enough. Carving out space for each track is important, so you don't have frequencies competing with each other. Ozone and Harbal have great tools that compare your EQ to the pros and can help correct problems in the frequency spectrum.
The average listener doesn't hear music through professional monitors, but through cheap car speakers or a boom box. I bought a pair of very cheap, crappy desk speakers and use them to do A/B comparisons against my monitors. If I can get the mix sounding good on the crappy speakers, it usually works everywhere else.