Volume Space
Or sometimes called vertical space. The goal is to be able to hear everything at a very low volume as well as the intended volume. When you turn down the mix to listen for each component and one is missing, just nudge the missing component upward in volume slightly until you hear it as you're listening at that lower level. Then check the mix at the desired volume. No hard and fast rule on this one. Just keep in mind that if a component is unintentionally louder than the primary (main vocal or lead guitar, e.g.) then it will be distracting.
My weakness here is that I love to sing, and I love to hear great harmonies. When I mix, I tend to make the background vocals too loud. If it's your desire to have the main vocal above everything else, or at least to be heard clearly, then listen to all vocals in the mix at a very low volume level to hear that the lead vocal is above the rest.
Use tricks like almost closing the door and walking down the hall. Listen to see if you can hear each component. This is also a very good final mix trick. Many engineers that have written books and articles on this topic simply say that they mix music at a relatively low volume level. They often say that the near field monitors are for mixing. The large speakers in the room are only to impress the client.
Also, a very important factor in all of this is the longevity of your hearing. Keep in mind that your ears will get more tired and your mixing sessions will be relatively shorter at higher volume levels. I say this because when your ears get tired your brain has more difficulty in making mixing decisions.
Next up in the series is
Tips for Audio Mixing: Effects Space.
