The questions to ask about your mix

Number 1
Can you hear everything in the mix?
A key part to a mix is the ability to hear all the instruments and voices in a song. If you cannot hear all the assets that you want in the song a couple of solutions are to turn the volume down on other tracks to bring out the hidden tracks, or you can easily pan tracks to fill the stereo spectrum to also bring out the hidden tracks.

Number 2
Can you hear the vocals in the mix?
The human voice can rage anywhere from 82.407 Hz to 1396.913 Hz. (See how the vocal frequencies breakdown in next section) There are many instruments that can collide with the vocals in the mix. An easy solution to this is to find the instrument that is in conflict with the vocal and create a small dip in the instrument EQ to clear up the vocal track a bit.

Number 3
Are the vocals to sibilant?
Sometimes you might get a vocalist who likes to sing with a heavy "s" or "sh", or sometimes you might get assets that had a bad microphone choice. In any case you can use a de-esser, which will compress the 3kHz-20kHz range, or you can cut the high frequencies around 7kHz-10kHz.

Number 4
Are the vocals coming in to hot or cold?
If you vocal track has volume issues the best tip I can give is to automate your vocal track to the volume you want. Some people will say you can throw a compressor on the vocals, but if you automate you can save the compression for the master track and not have to worry about you vocals getting over compressed.

Number 5
Are the vocals sounding squashed?
If so this is an easy fix. You are applying to much compression. Reduce the ratio or raise the threshold so the gain reduction is 6dB or less.

Number 6
Do the solos blend with the song?
Guitar solos should be as loud as the main vocal. Riffs and fills should be quieter during vocal rests so the do not become too distracting.

Number 7
Do the harmonies blend?
Harmony volumes are usually set below the lead vocal enough that the lead melody is clear.