My students ask me, "How do I mix?" I say, "Make yourself two different people." One is the professional mixing engineer that knows about technology and every sound that is in the audio spectrum of the song that you are mixing. You have to be able to hear everything clearly that is on tape. You have to pick them apart using your ears, your head, and your mind. Then you have to say, "Okay, let's analyze the mix. Let’s listen to each individual piece separately and really focus our concentration on what makes a good mix." Then you also have to think, "What would someone that doesn’t know anything about technology or music think?" Try imagining that they're in the next room, they hear a song and they say, "Oh, I like that song." If you have satisfied both of those perspectives, you have a good mix.
Reminds me of Stav's argument for grouping left- and right-brained activities in Mixing With Your Mind, an excellent book with lots of good tips.
Although this one isn't in there -- maybe it'll be in the follow-up?