Andy covered most of what I woulda contributed, (thanks, again, Andy!) but a couple of things I could expand on, a little (or a lot)
Arrangement is the first, most important step; you want a situation that doesn't
need fixing, so you can get on with your
next masterpiece, right? When you're choosing your sounds or setting up your mics, you should
already be thinking about how things are gonna fit. This requires mindful practice and a good deal of experience, but listening to lots of well-mixed music
in your mix environment will make it happen faster.
I'd suggest that you pan
nothing until you get your mix working in mono, and the time-tested method for this is subtractive EQ; always cut first, because if you boost, you're giving away precious headroom. Jimmy Page said distance makes depth, but if you can't move a mic, you can simulate it by rolling off highs, pushing back mix elements that don't need to be featured. In this way, when you start moving parts out of the center, your mix will feel not just wider, but
deeper.
The Melda thingy? Might be just the ticket, if either your ears or your monitors suck;
preferably, you just stick a single band of the parametric EQ you undoubtedly already own on the 2-bus,
boost 3-6dB with a relatively narrow Q (bandwidth) and s
weep it up and down to see what kinda nasties pop out.
Listen for conflicts. (go ahead, close your eyes and turn the knob while the music plays) If you think you hear some, solo the tracks in question and
cut appropriately. You'll already have the frequency sussed out, so then it's a matter of adjusting the depth of your cut and the Q. Of course, there can be more than one frequency in conflict, but this is basically how you find them
without visual help. You can do this on individual channels, too, but don't forget to listen to your edits in context, to make sure you didn't just bring your track or tracks
into conflict with
something else.
The other big reason to cut, rather than boost is a concept called masking; certain frequencies,
usually low frequencies can mask the ones that provide punch and clarity. 400Hz (give or take; it depends on the individual instruments) is just a nasty, woolly hog that steals headroom and covers up stuff you wanna hear.
The classic example is balancing a kick drum against a bass, where you might use a resonant filter to bring out the foot of the kick and a low shelf to kill everything below about 50Hz, (trust me, on a kick drum there's almost never anything down there you
want to hear) add another pointy boost to bring out enough beater click for definition and scoop out somewhat between to leave a donut hole for the bass, which might still get a cut of its own, around 400Hz to take the fur off, though if the arrangement (there's that word again) is sparse, you might want to leave
some fur.
If you simply MUST buy another plugin, there's this:
http://forums.taxi.com/topic131818.html
You might also find
this helpful, though it's slightly off-topic.
Interesting how it all works behind the scene. I havent a clue. I'm just an end user. 
