A great master relies on a great mix. Mastering's true purpose is to bring multiple songs in the same sonic space - to make them sound like they belong next to each other.
Create inter-song cohesion
Dynamics and loudness management
Fix mixing errors
Make your tracks louder than everyone else
Turn crappy songs into good songs
Restore lost content
Get a great mix for mastering
Levels should be balanced. The central focus of the track should stand the most forward, but not so forward that it blocks the view of other elements of the song. Work Backwards from there.
Utilize mix buses and side-chain compression.
Panning should be diverse and smooth. Widening tools create width that does not usually translate well. A lot of playback systems are mono and these systems expose widening tricks. The best way to achieve a truly wide mix is by spreading elements across the stereo spectrum with panning. A good rule of thumb is to keep anything below 100 Hz in mono - kick drums, sub bass, etc. This is especially important for vinyl releases! Higher frequency items such as shakers, hats, and atmospheric pads generally tolerate stronger panning and therefore provide more width.
Always check your mix in mono.
Equalizers are used in two ways: creatively and critically.
Creative EQ includes extreme boots and harsh cuts. It's a tool for sound design and often very useful.
Critical EQ is used as a mix tool. This includes high-pass filters to remove unnecessary low-end or gentle bells/shelves/filtering to fit pieces of your sonic puzzle together.
Keep these two types of EQ separate and remember that both styles can and should be used.
Clean up the recordings. Gating helps lower the noise floor and manual editing should be used to remove clicks and pops. De-essers are great for controlling sibilance and high-pass filters reduce rumble. Outfit your recording space with a good quality interface, microphone, and acoustic treatment or a reflection filter to do a lot of the heavy lifting before you get to the mix stage. Check for 60 Hz hum from dirty power.
Vocal dynamics pro-tip: Use a 1176 to shave the transients and an LA-2A to level out your recording.
Don't 'pre-master.' Avoid limiting or compression on the master channel.