When people ask me what the secret of mixing is, I answer: “Use as few (well recorded) tracks playing at once as possible, and you are golden.” Yes, because mixing is a job that gets exponentially more difficult the more tracks you are dealing with. This is due to the fact that not only each recorded instrument requires its own share of attention (dynamics/volume control, EQ sweetening, placement in the stereo field, spacial depth, and even effects if you wish), but also because each extra part adds a new variable in the mix, interacting in often unpredictable ways with the other sounds.
Busy arrangements invariably end up creating crowded midrange frequencies between 250Hz and 3Khz. That is where pretty much any instrument has its “body” and its “clarity” or “bite” (see picture). Sounds like strummed distorted guitars and keyboard pads make things even harder because they occupy an enormous frequency bandwidth, behaving like sonic blankets. Frequency interference between sounds creates phase-related conflicts that end up making some parts in the mix sound weak or bad. This is why a track that sounds glorious by itself may sound pathetic in the mix: Surrounding sounds are sucking frequencies out of it.
But paradoxically, “sucking frequencies out of the tracks” is exactly what mixing engineers do to make their mixes sound better: They pick and choose the “indispensable” frequencies for each instrument (normally a body frequency and a clarity one), maybe boost them a little with EQ while cutting more drastically what can be sacrificed. In particular, make sure you keep the mid low frequencies under control with a low shelf EQ, and just cut out altogether bass frequencies for instruments that don’t need them with a hi-pass filter tailored to each instrument (roll it back and forth until you hear you are cutting too much).
This, of course, limits the amount of conflicts between the tracks, but the more tracks you have, the harder it gets to carve out some room for everything. Fortunately, the stereo field also helps: If two instruments are sitting in the same frequency range, they can be placed far right and far left.- PDG
(In the picture: some instruments have no bass, some have no highs, but almost all of them have mid frequencies).
Here’s a useful video about this topic courtesy of Recordingrevolution.