Here is a couple of things that has helped me a lot with battling the low muck that can so easily happen in a mix.
First, I like mixing with an analyzer running on my control room output so that the analyzer is always showing me what I am hearing (steroe mix, solo's, aux cue's etc...) I do not rely on the analyzer, but helps me to visually confirm things that I may already suspect. It also becomes more and more useful on longer sessions when you start getting a little more tired and your ears may be starting to fatigue a bit. Sometimes it will show you how to battle the "stacking" issue of multiple tracks as well. The analyzer I use also has phase correlation meters and shows both peak and RMS levels as well as stereo imaging. All of those tools can be used to ASSIST in making a wider, smoother, deeper mix that has a better overall spectral balance.
The second trick I have learned that has not yet been mentioned is panning. There is absolutely no reason why the bass and kick have to bothe be panned dead center. Often times the only thing in my whole mix that is panned dead center is the lead vocal. Generally, I like my kick and snare a touch to the left side (like even just 2%), and my bass to oppose it on the right, along with the hi hat mic which often helps to center the snare back up. This slight bit of panning does not seem to drop any of the power of the low end punch, but really seems to clarify both the kick and bass to where they are easier to comp and EQ to get to fit each other a little better. It may sound a little odd, but try it sometime. Try panning a little further even just to see how it changes things so you know when to use and when not to use this type of trick.
One other thing I have learned from some really good engineers is how to use just a touch of reverb on the kick drum. Using a good short verb (1.2 or something) can really add realism, clarity and volume to the kick without it "poking out the front" of a mix so badly. You will find that this trick also allows your peak values of your mix to be a little less determined by the drums so you can focus on keeping a powerful vocal.
