One of the more difficult aspects of mixing is overcoming frequency masking.  At least that’s been my experience.  I’ve seen a couple of you tube videos but I’m looking for more.  They’ll show reducing one track by 2-3 dB in a frequency range and boosting another track in the same range by that amount.  It seems to me there is more to it than that.
 
My preference is usually a dense mix.  I constantly think of more I can throw into an arrangement and build it up.  But, I keep losing clarity in the previous tracks (to a degree).  How in the world do professional mixes sound so spacious and 3 dimensional?  I know, I know, because they’re professional.
 
What works for you?
 

aggressive roll-offs,
aggressive high-Q cuts.....
 
understanding what frequencies that will become trouble..... BEFORE YOU TRACK!!!
 
that is key.
learn your frequencies on every instrument you are tracking.
 
make room for everybody.
if something needs to dominate at 100hz, then get rid of that frequency everywhere that it does not really count.
 
 
using visual tools will help folk who are just starting out, to recognize 'bad' frequencies, or frequencies that are building up.
 

and stop watching utoob.

Recording Revolution does explain it here:
https://www.recordingrevolution.com/mixing-with-eq-carving-eq-holes-part-5-of-7/https://www.recordingrevolution.com/mixing-with-eq-carving-eq-holes-part-5-of-7/

The issues are sometimes more like Bat mentioned. Though masking is a real issue. Sometimes a small rolloff isn't enough...gotta get more aggressive. 
 
I think the visual tools to identify masking are good for everybody. I don't believe anyone can hear good enough to correctly identify all of it. At least, I see it as a time saver. I could twiddle with EQ for an hour or look at a visual and see the clash right away.

I have the Melda MMultiAnalyzer when I need help with frequency clashes. 
 
Besides using your ears, it's really cool to see the issues.
 
https://www.meldaproduction.com/MMultiAnalyzer
 

There are many different freely available online frequency charts - many are interactive, that show the frequency ranges of all kinds of instruments, and you can combine that with a spectral analyzer to figure out which of your instruments have frequency conflicts.
 
From that, you can figure out where to notch out specific frequency ranges from competing instruments, to make more room without it all sounding too thin.
 
Bob Bone
 

If you want to prevent masking, start at your arrangement ... the denser you want your mix, the more important the arrangement is. For example if your piano and your bass collide because the piano is in the wrong octave, no EQ will be able to fix this. You will end up EQing most of the piano away to keep the bass there ...

FWIW I never use analysers for this task. I go by bringing elements into the mix in order of their relevance and listen for coloring/masking whenever I add an instrument ... so if I add e.g. a piano and the lead vox gets pushed down or more often coloured, the piano needs EQing so that the more important vox can stay as is ... it's very interesting how much you can cut from an instrument that only supports in the background and still have it sound good ... Just don't push the Solo button on those EQed tracks in the back ...

RobWS
I constantly think of more I can throw into an arrangement and build it up.
 

Stay in mono when you throw it in, if it clashes, then re-consider if it's needed. Or use it in one part and what it clashes with in another.

 
There's a brilliant tool for addressing this problem and there's a deeply comprehensive article about it that you can read here:
 
http://soundbytesmag.net/spectraldynamicsdynamiceq/