Mixing in isolation is dangerous.
What we mean by this is that it is very easy to get wrapped up in producing sounds that sound great while you are mixing, but that’s only relative to that mix. It’s a common (albeit sad) occurrence to work on sounds until they sound absolutely great to your ears, but then when you hear them played back to back with another song, you realize just how bad the mix sounds. You can greatly cut down on this occurrence by using reference tracks while you mix.
A reference track is exactly what its name implies: it is a professionally recorded track that you use to compare your mix. So if you are mixing, say, drums and you think you have a really great sound, listen to your mix in comparison with your chosen reference track to see how your mix stacks up. Often this will reveal undesirable sounds you didn’t notice in your mix.
Of course, this works for all instrument tracks, as well as your overall mix.
Needless to say, you’ll want to use a track that sounds great. Pick a track that you admire for the quality of its production and engineering qualities.
More than that, you also want to pick a track that sounds similar to the sound you are trying to achieve. Therefore, if you are mixing an Americana track, it would be rare that you’d want to compare it to a hip hop track (though there are exceptions to every rule, of course).
One question that pops up is, ‘if I am mixing a track that is obviously not mastered yet, why would I compare it to a mastered track?’ Well, there are a couple of reasons for this.
One is that you are unlikely to find a great sounding unmastered track. Since almost all music released is mastered, it would be very difficult to come across your favorite song in an unmastered version.
Second, even if you could reference an unmastered track, you probably don’t want to. The reason for this is simple: the best mixes produce the best masters. So if your unmastered mix sounds nearly as good as a mastered reference track, you are going to end up with an even better final product.
This is not to say you should slap a major compressor on your master track to emulate the loudness of a mastered track. Certainly mixes should be significantly quieter than a mastered track, as these result in the best mixes. You are referencing for the sound, not the volume, though you may want to adjust the volume between the reference track and your mix for better comparison.
There are many ways to do this, but perhaps the easiest and best is to simply import your reference track into a free audio track in the session you are mixing in so you can simply mute and un-mute the track for a reference. This ensures great comparison since all audio is coming from the same speakers and preamps (if applicable).
One note on this: be sure to use a lossless file for your reference so you don’t lose sound information in a compressed file (i.e. mp3).