Mixing isn't about setting all the dials and leaving them where they are for that perfect mix, things move, you need to control things a little and move them around so they get focus or defocused in the mix. The final mix may sound more static than that, but that is the art - to produce something that is glued together well.
The excercises we were given in audio engineering were how to not only disect pieces of music but also training our ears to identify frequencies. You can think of these excercises as practicing your scales and techniques so when you play your instrument, your fingers go to the right places at the right pressures to create the piece of music. So to must you train your ears and mind to understand and hear things - once you really start analyzing things you will not only learn from other people's mixes, but also identify areas in your own mixes where you can improve.
A good way to learn how to mix is to study a similar track to one you are trying to create - break it down into a square as a cross section of a cube from the top down. Closest face to you is front of the mix, left, right.
From this, analyze your reference track and figure out
1. Positioning of instrument in stereo field. (L/R)
2. Tone of the individual instrument (color) - bright, dull, thin, thick.
3. Loudness of each instrument.
4. Depth of the instrument (wet/dry reverb)
5. Draw directional arrows when things move around in the mix.
You can then take this visual diagram and start to apply it to the similar piece of music you are trying to mix.
Practice using a 24band EQ on your favorite piece of music. Listen to those - boost them by 3 or 6db - Have someone else do this or automate in your DAW and see if you can hear those frequencies correctly. Training your ears to hear frequencies really helps with mixing and pinpointing issues and analyzing other pieces of music.
There's a great book out there called Mixing With Your Mind which has some fantastic easy to understand analogies and techniques for mixing.
Most of the tools - compression, eq, reverb to shape your instruments into the mix will just be a lot of practice. Took me many many years to fully understand how various compressors worked and how to use those to shape things like snares, kicks, hats..etc. I'm still learning, and you always keep learning - mixing is definitely one of those life long things and as you practice you will learn how to mix better over time. I have been mixing now for over 10 years and still learning, practicing!