Your mix can be thought of as having two spaces where you can place your instruments.
The two spaces are the Front and the Back of your Mix.
When mixing, you want to decide whether an instrument will be positioned up Front, which means it sounds like it’s close to the listener or if it’s positioned in the Back, which means it sounds like its far away.
The best way to really sensitize yourself to whether things in your mix are close or far away is to listen with your eyes closed. This is because when our eyes are open they tend to dominate our perceptions and actually decrease the sensitivity of our hearing.
Here are some simple distinctions I’ve made between the Front and Back of my mixes:
Instruments that are up Front:
Instruments that are in the Back:
You’re mixes will instantly gain more depth and clarity when you make a practice of being definite about whether you want something in the Front or in the Back.