I'm a firm believer that most "mixing" happens in the arrangement/orchestration stage. Sometimes the best thing I can do for a song is delete a part that doesn't need to be there. It's so easy to get carried away with layering, with today's technology essentially eliminating almost all technical limitations of recording and composing. But just because you can easily stack a hundred tracks on top of each other, doesn't mean you should. Restraint is an art form; the notes you don't play are every bit as important, if not more so, than the notes you do play.
More and more I'm learning that trying to fill every possible hole in the sonic spectrum just leads to mush, and ultimately, an endless "tail-chasing" cycle with EQ and compression and all that. The more empty space you have, the more powerful each note is within it.
When you've got dozens of synth plugins, and two hard drives full of sample libraries, sometimes the greatest challenge is refraining from cramming all that awesome stuff into every song. Tasteful, well-recorded sounds, when mixed with other well-recorded sounds, should ideally not need any EQ at all. In theory, anyway. If you're trying to carve holes to fit more stuff in there, maybe you've got too much stuff in there already.