I bet you didn’t know what it was. Because it’s not something you forget to do; whether accidentally or not. Nope. It’s something that you do. And you shouldn’t. At least not by default. Confused yet? Don’t be.
Don’t do what you don’t need to do
The biggest mistake you can do is to do something to the music that it doesn’t need. Changing things for the worse; just because you have heard or read about others doing it. That’s a very common approach to mixing, especially nowadays, when information is flying around like the queen’s fart in a beehive. So wait a minute, do I mean you don’t need to break out that hi-pass filter to make room for the bass? Or to do that weird cut-one, boost-the-other shit on the kick and the bass line? Or that you don’t need to compress the living crap out of the overheads? Because there are no overheads?!
Yep. Imagine a jazz band, playing their stuff into a couple of well chosen and setup microphones. Live, with no overdubs. Or imagine a bluegrass band playing into a single omni mic, playing harder or softer and go back and forth. To pretty much make their own mix on the fly. At least part of it, the other part being how the instruments blend naturally.
Not only for purists
You getting it yet? Hope so. What?! You are not a purist? You got lucky this time. I forgive you my friend. Because the truth is, even if you are a one man band kind of musician (let’s not use the popular “producer” term for this, please), it still works. It doesn’t depend on genre, it depends on a well thought out arrangement, and careful recording/tracking.
If you know you’ll have a bass line, no matter if it’ll be played on a bass guitar or a MIDI keyboard, you can build the rest of the arrangement accordingly. For example, play your rhythm guitar tracks so you don’t hit the lowest string(s), at all, or play higher up the neck. This will prevent you from having to EQ out the low end of the guitar later, in an attempt to make room for the bass. And if you know you’ll have a busy kick drum, just make sure you don’t make the bass line equally busy. With thinking ahead and arranging the instruments apart both rhythmically and tonally, your only task during mixing will be to balance their levels. Or not even that, if you’re lucky/careful enough. And the elements will sound bigger and more natural, because you didn’t have to make them smaller just to help them coexist.