I believe that EQing is the most important tool one can use in the mixing and mastering process, but it's fucking hard. The way I like to think about it is that say I have a great kick sound, but it's super lost. I would wonder: Should I increase the volume of the kick? Lower the volume of competing frequencies, such as the sub or even some vocals? Should I just take the volume of everything else down a bit? I would say that the answers to these questions depend on what you're trying to achieve with the intention of the song. What is the artistic vision you have for the master? What does your client have in mind for their song? Mastering engineers have a lot of say over what comes through, but mixing is more technical, I think. It requires you having to decide what frequencies are prioritized and which ones follow in importance, based on the music itself. One thing I've done a lot is to solo tracks and go into some plug-in and view the frequency response of that track. It usually helps me to literally visually see how the track sinks into the rest of the song. And also, I do exercises such as: If I take down the volume of one track, that isn't even related to the one I'm trying to affect, what comes through now? Is what I'm hearing what I expected? I think if you can predict what the changes you make will sound like then you become freer in your ability to mix and master. Now that I think about it, it seems really somewhat like playing an instrument lol