Some things I've picked up along the way, although I still need a lot of work on the said subject...
1.) It's much better to subtract frequencies that are undesirable than to boost the frequencies you like. For example, in a bass drum, when you hear it, you might not want the trebly stuff in there, so to accent the bass frequencies, we cut the high frequencies. This isn't always the case, but is a basic example of listening to your mix, and finding what frequencies you don't like in a mix.
2.) That actually just brought up my next point, LISTEN TO YOUR HEART! erm, mix (poor attempt at comedy :( Anyways, really, listen to the mix. It's important because it helps to know what your getting into before you actually start mixing. You might not have to EQ something, and that's always good to listen for beforehand.
3.) Loud isn't always good. As artists, we want a full sound, rich in the different frequencies, and a clean sound so there isn't a distortion where it's not needed. If a mix sounds balanced at any volume, that's at least one way to know that most things are going great in the mix.
4.) When mastering (after the mix is done) if you are having difficulties in a certain frequency range (treble high frequencies, middle frequencies, or bass frequencies) it might because there's a problem in the mix. Go back and look at different instruments to find where the problem might be. If a bunch of instruments are mixed to the same frequency characteristics, it can hurt the effectiveness of the mix. Look for gaps in the mix and fill them with an instrument/synth that will sound good.
-That's all I've got off the top of my head, I hope it is helpful for everyone.
