mettelus is on the money there!
Whenever I sculpt the tone of an instrument, I make a copy and change it, if the track is going to get solo'd in a breakdown or section of the arrangement where it warrants a different EQ. Obviously you can apply the EQ to the parts of a song all in one track, but I find it easier to mix with them on separate track faders.
Michael

mettelus
 
As new elements come into the song, it is common to have that riff be lowered to give room for new elements. The listener still hears it.

Zeps "Ten Years" Page does this well.

There is a lot of info available on how we "hear" the full range of an instrument (like a octave baritone guitar)even though it "isn't all there", meaning you can EQ/sculpt away quite a bit and it will still sound/be perceived as "normal". This leaves you more room for the other elements of your mix.
Google psychoacoustics and EQ. There is so much info that will help you mix lots of tracks and be happy about your results.
Michael  

Never be afraid to change your arrangement to make instruments in the same frequency range work better together. Try and rearrange them so they're not all playing at the same time. At first this sounds like a compromise you shouldn't have to make (especially when you are emotionally invested in the music you've written) but in practice it often works out very well and creates a sound which is more musically engaging. 
 
The ear loves to pick out a single line from instruments that are entwined around each other. I do this a lot when working with MIDI parts. You can work with multiple instruments in the piano roll at once and see visually how a single line emerges from instruments which take it in turns to play. So you might have the first instrument play a couple of notes, then the second takes a few, then the third etc. When played back, the ear hears a single melodic line which sounds very interesting by virtue of the fact that it's being strung together very cleverly from multiple parts. It creates very pleasing textures, and is a lot easier to mix because none of the instruments in question are playing at once.

Some excellent stuff here...
 

mettelus
Take a look at "Mirror EQ"... Dan Worrall has a really nice video on this (is for a plugin, but the presentation is exceptional, and universal). https://youtu.be/kSNYBbPAvKE

 
I often return to this video, being a Pro-Q user... It's an excellent piece of reference material 
 
 

pentimentosound
It's true of vocals, too. I remember tracking a girl's 3 part harmony and I had to cut the sibilance on 2 of them or it sounded like a whip every time she sang an "s". Talk about build up! Sheesh! Leaving the foreground one alone sounded most natural. I only dipped her EQ on the "s" parts. It was my first serious de-essing mess!
Michael 

Hah!
 
Back in the old days it was common to get the singer(s) to omit all 's' & 't' sounds when overdubbing.
 
Sounded ridiculous on it's own, but the song benefited immensely

Right, Jonesey!   I suggested it to her, but she was new to recording and the headphones seemed to be enough of a distraction, without more suggestions from me! LOL
I was listening to a Shania Twain track, yesterday "Get You Good", that Mutt Lange is barely singing the words to. It sounds like he's singing "gi ya guh" vs enunciating "get you good".  Just enough sound to imply the words and yet it makes the BG vocals fit in the busy track. I learn something from Mutt every time I listen to a track of his!
Thanks mettelus that is a very good example of EQ'ing things.
Michael

SOME EXCELLENT TOOLS, TO BEG BORROW OR STEAL (OR JUST PURCHASE!)
 
WAVES PAZ
SPAN
HAR BAL
ReFuse Software
FreqAnalyst - Blue Cat Audio
Vertex DSP
Seven Phases Spectrum Analyzer
schwa schOPE, by Stillwell Audio
Nugen Audio’s  Visualizer
Melda Productions’ Manalyzer
 
 
helps your ears get thru a somewhat steep learning curve.
 
 
when you have buildup,
it's good to go back to individual tracks and WATCH what is happening at certain frequencies...
 
you can DECIDE (yes, decisions) which tracks need certain frequencies turned down that are not as necessary for that track, as the next one is.....
and LEARN how to mitigate frequency buildup.
 
some folks have magic ears,
and can just simply hear this happening.....
others can use a little more help (like me)

I have one or two spectral analyst things and don't forget so does Sonar Platinum in the mixer console.