Most recording engineers spend a long time learning the fundamentals of mixing. For sure, instrument balance, frequency balance and dynamics control are important, and you've got to have those in the first place for a mix to be any good.
But for a mix to be great, you have to go beyond that. Here are the three keys to a great mix:
- Figure out the direction of the song. For a mix to be great, it needs the perfect direction. In other words, if the song has an arena rock feel to it, you don't want the direction of the mix (the sound) to be intimate and in your face. Likewise, if the song is soft and sensual, huge bombastic reverbs won't be the right choice either. The songwriter and the artist usually have the best idea for the correct direction, but don't be afraid to experiment either.
- Develop the groove and build it like a house. Every song, no matter what the genre, has a groove. The groove is the pulse of the song and how the instruments breath with it. For the mix to be great, you've got to feel that pulse, therefore you have to find the instruments that supply that groove, starting with the most important (usually the bass and/or drums in most music, but not always). When you find the instruments that supply the groove, then you build your mix around them.
- Find the most important element and emphasis it. This is usually the vocal or lead instrument but not always. Sometimes it might be an interesting rhythm part or an instrument that's playing a hook or has a hooky sound. Whatever it is, you've got to find it and emphasis it, because that's what sells the song.