EQ - when to boost and when to cut
You have probably noticed that the gain controls of your EQ have minus settings as well as plus. If you haven't gotten round to using minus settings yet, that makes you one of a million learning sound engineers who need to discover this powerful facility.
When you are mixing a track, you will undoubtedly find that it falls into one of two types - cluttered, congested, difficult to make sense out of; or sparse and difficult to make the instruments gel together.
If you find your mixes positioned exactly in the middle of these two extremes, then you had better go out and buy a lottery ticket because you're naturally lucky!
If a mix is congested, then what it needs is de-cluttering. What is causing the congestion is very likely to be that many instruments are competing for the same bands of frequencies. So solo each track in turn and use the EQ to ease off frequencies that are not really important for that instrument, then boost a little the frequencies that are important.
Do this for all of the tracks in the mix and when you try again to mix them all together, you will find that the congestion has magically gone. Your mix will be crystal clear.
If you have the opposite problem, that the mix already sounds too clear, almost clinical, then you need to apply reverse tactics.
Once gain solo each track in turn, and use mid-range EQ boost first to identify the frequencies where the instrument is strong, then cut those frequencies. The reason why you should boost first is that the ear finds this easier to pick up on - you can hear the problem frequencies easily, then correct the problem.
This is not quite the reverse of the 'congested mix' technique as you don't have to boost unwanted frequencies. That would be overkill.
But once you have 'toned down' each instrument in your mix, you will find that they all sit together much more easily.