Your Ears' Natural Compression

You may not be aware that your ears (along with your brain) have a built in compressor. When the volume is high, your ears "go into" compression mode. Understanding little things like this about the incredible mechanism of sound perception gives a creative engineer ideas about how to use them to our advantage, and this is a good example. Monitoring loudly feels good (especially if you're working with talented musicians!), but if it's loud enough for your ears to go into compression, you're not hearing the actual volume balance of the mix, because that natural compression effect is "flattening" your perceived mix. So whenever you're doing a mix session, make sure to occasionally turn the volume way down. Any elements that are too soft or too loud will tend to become immediately perceptible.